FINE OIL FINISHES FOR THE INSTRUMENT BUILDER
By Bill Woods
I am willing to admit that, when it comes to instrument building, I am an inveterate experimenter. I will even admit that 75% of my experiments are failures, more or less, except for what I learn in the process. But, the handful of significant successes I have had seem to render the failures inconsequential, especially considering the improvements in my instruments (I build classic guitars).
In this article I would like to describe one of my successful methods - instrument-quality oil finishing. My finishing method has been developed over about ten years of experience with many different wood species, and is specifically suited to instrument finishing, where the finest surface treatment is required. I do not claim that my method is quick and easy; however, it is considerably easier than the traditional surface finish methods varnish, lacquer, and French polish.
Briefly, I would like to list what I feel are the most important properties of an instrument finish:
1. The least detrimental effect
on the tone of the instrument.
In all my experimenting, I have
never found a surface finish that satisfied me with
respect to the above list. My best results have always
been achieved through oil finishing. In particular, the
oil finish I will describe here excels in three
properties: it has very little effect on tone, it wears
very well with respect to appearance, and it is very easy
to repair. I generally prefer an oil finish in terms of
appearance - I don't really like a glossy finish (even
though I well appreciate the work behind it). Instead, I
like the polished glow of a first-class rubbed oil finish.
Dive To Bottom
Before I enter into the details of my method it will be useful to enumerate the general requirements for producing good instrument-type oil finish:
1. Careful surface preparation
Of all procedures in oil finishing, surface preparation is the most important. The reason is clear: in a properly done oil finish the oil is mostly in the wood, not on it; therefore, the surface of the wood itself is the primary determinant of the quality of the finish. Let me emphasize - never start oil application unless your wood is as flawless as possible - follow this rule and you will save yourself a lot of effort.
The sequence I usually use in surface preparation is as follows:
1. Optional special sanding
Now I want to point out that among instrument makers there are both adherents of "scraping" and adherents of "sanding." There is a perennial argument over which method is "best." Since I am naturally a skeptic, I believed no one - I experimented. My own conclusion at this point is that any instrument maker who doesn't use both scrapers and sandpaper is severely limiting his ability to create a fine finish. Scrapers excel for particular purposes; sandpaper does what a scraper cannot do. Using both you can conquer almost any surface preparation problem.
One of the last operations on a guitar before finishing is scraping out the binding. It Is critical that in leveling the binding you don't create problems that will show up in later finishing. This means you must pay careful attention to achieving a smooth continuous contour on the binding surfaces, and must avoid damaging the soft conifer top. Use a piece of masking tape on the non-working end of your scraper. When doing the final flushing of the binding to the top, take care not to bear down on the scraper or you may "roll up" the fibers of the top - a nasty defect that will take a while to scrape or sand out.
The problem I have in scraping the binding is due to the fact that I like to use solid flamed wood binding (no white line). While I do avoid the headaches of white lines, I have to deal with the pronounced tendency of the scraper to follow the wave of the grain, resulting In a rippled binding surface. I counter this effect largely by trying to work the scraper at many different angles over the binding. In most cases, this measure alone is not sufficient - in order to achieve a really smooth contour I have to use 150 or 220 sandpaper adhered directly to a small hard sanding block. A hard sanding block, used with care, will level this kind of defect. This is really the only kind of situation in which I would use sandpaper in contouring, before I begin my regular finishing sequence. Another place where you can use this technique is on the sides at the waist. It is very difficuit to angle the scraper in this problem area, which is especially prone to rippling. These "special sanding" operations are what I refer to in step one above.
At this point, you can begin step two, the final scraping operation. Your scraper should have the finest and sharpest possible burr - stone the edge and use the burnisher lightly. (Save yourself some trouble: use a European-made scraper, not a domestic one.) Scrape from a multiplicity of angles to start, then gradually shift over to straight over-lapping strokes with the grain to finish up. To judge the quality of the surface, continually check the reflected highlight of the wood, using a light source opposite the guitar. This method of using highlights to detect defects is indispensable to proper surface preparation. All common defects - tear out, ridges, rippling, brace "humps'; etc., can be detected, corrected, and avoided by this method. Be finicky - the better your scraping, the less you will have to sand, the more perfect your finish will be.
I find the scraper is most useful on the flat or single curved surfaces, top back, sides, and head. On double curved surfaces, like the neck, the scraper is less effective for final surfacing. In these areas, use extra care to eliminate as many ridges and narrow facets as possible before you finish. The various curved scrapers are very useful here.
Keep in mind that you may scrape any direction you wish except dead parallel to the grain (the edge of the scraper parallel to the grain direction). Under this condition, a scraper will almost always tear badly, especially on quarter-sawn wood.
Because a guitar top is particularly hard to scrape once it Is arched, I usually scrape to as good a finish as possible while I fabricate the top (before bracing). In this way, I only need to do touch-up scraping on the assembled top. Make sure that you round the corners of your scraper or you are certain to dent the top. If you have a "clean" dent in the top, you can often bring it back flush with this steam method: wet the dent thoroughly with water, place a damp double thickness of white cotton cloth over the dent; heat the flat head of a large nail with a flame, then press the flat face head directly onto the cloth. The resultant steam will lift the dent, while the cloth prevents staining of the top.
On the topic of scraping tops - I have had people tell me, "you can't scrape softwoods." This is nonsense. You can scrape most softwoods to a beautiful finish if you sharpen your scraper to perfection and use the smallest possible burr. In fact, you can use the scraper with just a clean, sharp, square edge and no burr at all. Then, while scraping, you must use fairly light pressure and numerous passes. Don't ever bear down on the edge or you'll have to start all over.
At this point, you will be ready for the step three sanding sequence. The first task is to round corners - on the head, fretboard, heel, binding, and soundhole edge. I use 100 and/or 150 grit sandpaper folded once or twice on itself. Folding the paper keeps it stiff, minimizing the danger of scratching adjacent scraped surfaces. Take your time on this operation - nicely rounded corners are a premier sign of fine craftsmanship.
Now you can start sanding the major surfaces of the instrument. Keep in mind that your purpose is not to remove lot of material, but rather to refine the scraped surface, removing scraper marks, chatter, and other very minor defects. If you have done your scraping correctly, you should be able to start with paper no coarser than 220; then you will proceed to 280 and 320. With skill, you can skip the 280.
Most people think of sanding as something very simple. Actually, sanding is a very subtle skill that requires patience and experience. To sand correctly you need the right materials and the right technique. As for sandpaper, I use no-fill aluminum oxide or silicon carbide - the highest quality I can buy. For a sanding block I use an "Artgum" eraser that measures about 7/8"x 7/8"x 2". I flatten the bottom of the eraser on sandpaper, and cut little sheets of sandpaper to fit (about 2"x 2-1/4").
All of the people I have seen sanding guitars were using big chunky quarter-sheet sanding blocks. Presumably, they needed a big block to keep the wood flat. I use a small block, because I have already scraped the wood flat and smooth, and the little sanding I do will not change that. Furthermore, by scraping I have totally circumvented the need for coarse sanding. For the fine sanding after scraping, I need sensitivity and control, so I use a small block. The Artgum eraser is just right in terms of size and hardness.
As for sanding techniques, there is a common pitfall that should be avoided - the "back-and-forth" sanding pattern. Virtually everyone, when given a sanding task, will rub up and back, end-to-end on the wood keeping the sandpaper in contact with the wood the whole time. This technique is very poor, since it resuits in at least twice as much sanding in the middle as on the ends - often the very ends will barely be touched at all. To sand evenly, it is necessary to sand in long separate strokes. Start at one end and glide the block onto the wood, move forward in a continuous stroke, and proceed right off the end of the wood. Now sand the same way on the return stroke. With this technique you will achieve superior resuits. Of course, there are many instances in which this exact procedure is not practical; but even so, try to sand in continuous discrete strokes.
In general, you will get better sanding results if you follow these pointers:
1. wipe or vacuum away the sanding dust between grits 2. use only sharp paper 3. use the "highlight" method to check for defects before you proceed to the next grit.
In the past, I have added 400 and 600 papers to my sanding sequence. I am ambivalent about these grits, because the closed-coat bond does not work well on wood. These papers clog very easily, especially on resinous exotics. At this time, I am trying to eliminate the use of these grits by substituting the polishing abrasive described below.
At this point, it would be possible to begin applying oil; however, to achieve the best possible finish, I suggest that you proceed to step four, wood polishing. This method is the newest addition to my finishing procedure. It is made possible by the availability of a new product called Micro-mesh, a very special kind of cushioned abrasive that is capable of incredibly fine polishing on most any hard surface. Micro-mesh is so effective that it is possible to polish raw wood to the point that it appears to already have finish on it. To be brief, it is the most amazing abrasive product I have ever used.
Micro-mesh is available from 1500 to 12000 grit, is washable in soap and water, and wears extremely well. For the subtleties of its use, you should read the excellent instructions that come with it. If you stop sanding with 320, you should start with 1500 or 1800 Micro-mesh; after 600 grit paper, you should start with 2400. You can go clear to 12000 grit if you wish, but it's a lot of work. My present method is to finish sanding with 320, then to polish from 1800 using 4/0 steel wool. The steel wool has only one purpose -the removal of raised grain hairs. Nothing works as well as steel wool for this purpose. There is only one 4/0 steel wool on the market that is consistently high in quality - Elephant Brand. Don't use anything else.
Micro-mesh works best on hard woods. On mahogany, spruce, cedar and the like I find that 3200 Micro-mesh is the approximate point of diminishing returns.
It takes some time to do proper surface preparation, but lf you do it right, the oil finishing itself is a breeze. There are many oil finishes on the market. I have achieved my best results with polymerized tung oil. I have tried many different brands of tung oil, and found most of them to be good; however, I still use the first brand I tried ten years ago, Jasco. It is readily available (try Standard Brands), consistent in quality, and reasonably priced.
With respect to the list of important instrument finish properties already mentioned above, tung oil scores very high. In addition, tung oil has very good drying power, and its penetration can be controlled. This second quality is very important for instruments, since an oil finish that penetrates deeply into the wood will almost without exception compromise tonal quality. For example, I would never use Watco oil on an instrument. It will penetrate clear through the top, back, and sides, and it takes two lifetimes to harden. I have heard guitars finished with Watco -the high overtones were severely suppressed in most cases. If you want a "live" guitar, stay away from Watco or any other thin "penetrating" oil finish.
There are certain distinct advantages to oil finishes. They tend not to show surface defects (nail marks on the top, for example), and they are easy to repair. (An oil finish repair has two steps - burnish the defect with 4/0 steel wool, then apply oil and wipe off the excess.) Finally, players like oil finished necks because they are "fast" - never "sticky" like a surface finished neck.
Now let's delve into oiling the guitar.
At this point, you need to decide whether you want to glue the bridge on the unfinished top, or wait till after finishing, in which case you will need to scrape away the oil where the bridge will glue. I have tried both approaches and conclude that neither method is decidedly superior. All in all, I prefer gluing the bridge on the raw top, because I don't like to get glue and water all over an oiled guitar top. (My bridges are finished with tung oil while still attached to the work block).
Now proceed to put on the first coat of tung oil. Filter the oil into a shallow container and use a white cotton or linen pad for application. Start with the top (to avoid possible color transfer from the back and sides), and apply the oil quickly to one quarter of the top - no more. Now immediately wipe off the excess oil with a white cotton rag (cheesecloth is good). Do not use a synthetic fiber rag; it will scratch the wood.
Proceed to finish the rest of the top, one quarter at a time, wiping off the oil immediately after application. Do not let the oil soak, and certainly do not thin down the oil before applying it - use it as it comes from the can. You want minimum penetration to avoid compromising tonal response. Furthermore, do not try to leave any oil on the surface, on this coat or any other coat; wipe off all the excess. If you try to build tung oil on the surface, you will regret it later when you are rubbing out.
It is very important to apply oil to small sections, wiping it off quickly; otherwise, you will not achieve an "open pore" finish on the porous woods. If you wait too long to wipe off the oil, it will thicken and not "wick" out of the pores into the rag; as a result, after repeated coats, the pores will be glossy, which looks bad on an open pore finish.
Continue with the above method over the whole instrument, including the fretboard. Do the neck last, so you can use it as a handle while oiling. Hang the guitar to dry.
While oiling, use some sort of white cotton glove on the hand you hold the guitar with (I use a cotton sock). Do not touch the guitar with your bare hands at any time during the finishing process. Until its completion, a tung oil finish may take fingerprint stains, especially on rosewood.
Each coat of tung oil should dry about one day. Rub gently between coats with 4/0 steel wool to remove any raised grain or surface debris. If the tung oil seems sticky when you start rubbing, stop immediately; otherwise, you will end up with a mixture of tung oil/steel wool "goo" in the pores. Let tung oil dry thoroughly before steel wooling.
If your surface preparation was good, you should have an attractive finish on most any hardwood after three coats. In order to maintain open pores, you should probably not apply more than four coats. On non-porous woods, five or six coats are fine.
You will probably not be satisfied with the sheen of the top after three coats. I have found that cedar and spruce are relatively difficult to oil finish. I often apply up to eight coats on the top to achieve a satisfactory sheen. This number of coats has very little effect on tone as long as you do not try to build a finish on the surface of the wood. Remember, wipe off all excess oil on every coat. In the process of gluing a bridge onto a finished cedar top, I have had an opportunity to examine the result of eight coats of tung oil, properly applied. I estimate that the total thickness is "003 - .005". Of course, much of this "thickness" is really penetration; the total amount of oil on the guitar top is actually very small. The result, I find, is a practically negligible effect on tonal response.
One more pointer in finishing the top - do not under any condition use heavy pressure when using steel wool; it is more abrasive than you might imagine, and can easily rub right through the beautiful sheen on your top.
When you have applied your last coat of oil, it is important that you wait for the entire finish to harden before attempting the final polishing. I would consider one week to be a minimum; four weeks would be ideal.
The polishing process is relatively easy. Go over the guitar carefully and gently with 4/0 wool - use light pressure for a burnishing action. If you run across any "beads" of tung oil, which sometimes form at the pores, do not attempt to remove them with the wool. Instead, take a brand new single edge razor blade and lightly scrape over the beads (at a negative rake) to remove them. Then continue with the wool. Be especially careful on all corners.
If you wish, you can polish further using Micro-mesh. This will work only if the oil is thoroughly dry. Start with 6000, then go to 8000 and 12000, all the while using fairly light pressure. Wipe the surface between grits.
When you are done with polishing, apply a coat of lemon oil treatment (I use Jasco) to all finished surfaces, using a white cotton pad, and let dry. This step Is important as it will satiate the surface's affinity for oil, and prevent finger-printing. Finally, polish the whole guitar with thoroughly cleaned burlap.
To care for the finish, use burlap for cleaning and polishing. A damp rag may be used on stubborn residue; wipe dry immediately. I have found that regular use of lemon oil treatment is really not essential to the longevity of the finish. A yearly application should be sufficient.
Personally, I recommend against the use of waxes, since I believe wax will slowly degrade a finish, whether oil or surface finish. In particular, wax application will ruin an open pore finish. Waxes are popular because they yield a fast, easy surface sheen. If you do a careful job in finishing, however, you will achieve a permanent sheen with the tung oil alone which totally obviates any need for wax.
When using tung oil for finishing, there are a few things to remember. First, tung oil has very good drying power, which means it will eventually gel in its container. Therefore, buy only pint containers and immediately transfer the oil to flexible plastic containers from which all air can be expelled before sealing (hiker's food tubes or photographic chemical "accordion" bottles work well). Then store the tung oil where it will not be exposed to light. Partially thickened tung oil can be salvaged by thinning with mineral spirits; but never use such oil on an instrument. Only fresh oil should be used for fine polishing.
With one exception, tung oil will dry on any wood I have worked with (which is unusual for an oil finish). I have never figured out how to tell the "non-oil" cocobolo from the "oil" cocobolo. On the "non-oil" variety, tung oil will remain tacky indefinitely (you can clean off the oil with mineral spirits). Fortunately, most cocobolo can be left unfinished but this would be unwise on an instrument. On anything but an instrument, I would just apply a "color coat" of tung oil, and rub off any slight stickiness with steel wool. For an instrument, I would experiment with a very fast-drying lacquer or shellac in light coats. On some cocobolo, however, even most lacquers will not dry.
I have recently been experimenting with an oil finish called Tru-Oil, which is intended for gun stock finishing. I do not know the contents of this product, but it strikes me as being a "bodied" oil finish, that is, an oil/resin mix. With respect to build and drying power, it seems superior to tung oil, and probably has better moisture resistance, too. In addition, it works very well on cedar. Before using Tru-OiI on an instrument, I intend to check into the resin content. I suspect it contains urethane, which has a reputation for compromising tonal response. Like tung oil, Tru-OiI will not dry on certain cocobolos.
I have also recently tried samples of 3M's Scotchbrite Cleaning and Finishing sheets. The Ultra Fine Type S (Grey) may replace 4/0 steel wool for many purposes, and the Cleaning and Polishing Type T (white) looks good for satin polishing.
Wishing all success in oil finish, I believe if you follow my methods you will be very pleased with the results.
Postscript: I no longer use steel wool for finishing. 3M's Scotch-B rite Cleaning and Finishing Sheet, Ultra Fine Type S (grey), is very effective for both final wood surfacing and rubbing between coats. Also, the Cleaning and Polishing Type T (white) can be used to advantage in some situations. The Ultra Fine can often be found in hardware stores. Both grades can be obtained through industrial suppliers. Having now finished a guitar with Tru-Oil, I can say that it is superior to tung oil for purposes of my method. It dries and builds faster, has better moisture resistance, and can be padded onto cedar to get a faster build. Finally, it seems to have no detrimental effect on tone. By the way, if you want to make your own lemon oil treatment, try 60 drops of lemon grass oil (from a health food store) In 50 milliliters of odorless mineral spirits (paint thinner). I'm experimenting with this mixture with good results.
Bill Woods is a classic guitarmaker, tool designer, and consultant from Prescoff, AZ. Bill has been a big help to us over the years with R&D and has several items that we carry in this catalog to his credit.
Micro-mesh is sold by Micro-Surface Finishing Products, Inc., 1217W. 3rd Street, Box 818, Wilton, Iowa 52778. It may also be available from a local vendor. Check with aircraft maintenance suppliers.
Jasco Tung Oil and Lemon Oil Treatment are made by Jasco Chemical Corp., Mountain View, CA 94042.
Tru-Oil is normally obtainable through gun shops, and is made by Birchwood Casey, Eden Prairie, Minnesota 55344. Santa Cruz Guitar, carved maple back, flat top.
On the backing of most sandpapers you'll find the following information:
1. Type of abrasive: garnet, aluminum oxide, flint, silicon carbide, emery. 2. The weight of the paper: A, C, D, and E. 3. Whether it's open or closed coat. 4. The grit number (120, 240, etc.), or the grit symbol (expressed in aughts such as 6/0, 2/0, 8/0, etc.).
Most luthiers know that progressively finer grits are used on the work as it approaches finishing, but not all luthiers know how to use these other features to their best advantage. Following Is a brief discussion of these lesser known features:
To refer to number 1, above - Garnet is popular among cabinet makers and furniture builders, partly because it's been around for many years, but largely because of its crystalline nature. As it wears it fractures into smaller grit with similarly sharp cutting edges and it cuts wood through a scraping rather than a scratching action. It's also a relatively inexpensive sandpaper.
Aluminum oxide is a hard, durable, more expensive abrasive, which is good for high speed sanding of hardwoods. Because the grit has less chip clearance it has a tendency to load when abrading resinous woods and soft woods. In our own use, we have found that spraying silicone on our thickness sander belt prior to sanding resinous woods such as Indian rosewood, more than doubles the belt's life.
Flint is the original sandpaper and though it is not very tough and wears quickly it is useful for removing substances which would simply clog a more expensive sandpaper. Use flint for removing paint, resins, sealers, etc.
Silicon carbide is considered by many to be the best all around paper. It's available in either waterproof black, or white paper. It's the hardest of the abrasives, though aluminum oxide, garnet and emery all have a greater resistance to wear. Silicon carbide paper does not load quickly because of the grits' greater chip clearance angles, but it does break down quickly on hard metals. It's a good paper for everything else, though, from softwoods and hardwoods to varnish, lacquer and other finishes.
Emery paper has the greatest resistance to wear of all the abrasives listed, but because of the grits low chip clearance angles and its tendency to fracture into dull cutting edges, it promotes heating. It is therefore best used with a lubricant and generally on metals only.
The weight symbols on the backing: A, C, D, and E corre- spond to the heaviness and the flexibility of the paper. (Cloth and fiber backings and combinations thereof are more for heavy duty purposes. "J" weight Is a relatively light weight paper and is somewhat flexible and "X" weight is a heavier weight and virtually Inflexible). "A" weight is for finishing. It's very light so that you can feel surface defects and it is flexible enough to allow sanding in small, narrow areas. "C" and "D" weights are heavier and carry coarser grit. These papers are not as flexible but can handle more pressure and are used with orbital or in-line sanders as well as in hand sanding. "E" weight is the heaviest of the papers and is found in roll form for use with belt, drum and thickness sanders.
OPEN & CLOSED COAT
"Open coat" paper is paper in which the grit doesn't entirely cover the paper's surface. Generally about 50% or 60% of the surface is covered by grit. Its best use is found in sanding (with light pressure) resinous woods, or other woods that have a tendency to clog. Not only do the open spaces of "open coat" paper resist loading but they permit you to flex the paper more easily when sanding Irregular surfaces.
"Closed coat" papers are completely covered with grit. It's best to use them on hardwoods or woods that don't clog the paper. They'll produce a smoother surface and will last longer, even under heavy pressure.
Some manufacturers refer to their papers by grit number, others by grit symbols (aughts) and still others by the rather arbitrary system of coarse, fine, finer, finest, most finest, super most finest, ad infinitum. The fine abrasive, grit #400 corresponds to 10/0, #320 to 9/0, #80 to 1/0 and so forth. The grit number is determined by the openings per square inch in the screen mesh through which the grit is sorted. #80 grit paper, for instance, carries grit that has fallen through a mesh with 6400 openings per square inch (80 X 80).
Remember not to go from a coarse paper to a fine paper. Use a medium grit paper in between, otherwise the deeper scratches left by the coarse paper will be very difficult, as well as time consuming to remove.
After final sanding it's a good idea to begin finishing the work at once. The raw wood can pick up moisture and dirt from the air, or the wood could be bruised by a nonchalant, bull-in-a-china-shop type person. So protect your work and finish it upon the final sanding!
Finishing is important for keeping dirt out of the wood, for bringing out colors and visual and tactile qualities, for preventing surface damage from heat and abrasion, and for keeping moisture exchange with the air to a minimum. But finishing is also important to the instrument builder as the choice of finishing materials can greatly influence the clarity, brilliance, and volume of the instrument.
Possibly a gap in our finishing section is the absence of a good varnish for guitar and dulcimer makers. We can readily obtain spirit and oil varnishes but these are primarily for violin makers, and are available through violin supply houses. The varnish that we had carried is no longer produced, and we were unable to find anything that would have been a suitable substitute. However, it was a tung oil based varnish that worked by polymerization, much like that described in Bill Wood's article, Fine Oil Finishes for the Instrument Builder. It is our suggestion that you give Bill's method a try. It will undoubtedly yield first class results.
For about two years we carried the Sherwin Williams lacquer which was a special formulation for the C.F. Martin Company. This was a pretty good lacquer but if sprayed any thicker than 6 mils it would develop cracks. This was more often a problem only around the neck. (Martin sprayed their guitar bodies sans neck.) After two years of experience with this lacquer, we concluded that the whole trick to the Martin finishing system was carrying out their entire process. This involved special vinyl undercoats, sealers, and retarders which were just too many chemicals for us to stock. There seemed more than occasional blushing problems with the Sherwin-Williams lacquer, too.
In the last several years we've carried a lacquer, which, if we were to judge only from its sales, is an excellent lacquer. We are now selling it to several customers In 5 gallon pails. One of the men here, who builds and repairs says it's the best lacquer he's used since a German lacquer became obtainable about 12 years ago. Charles Fox, (see article on Universal Wood Bending Machine) who headed the School of Guitar Research and Design for many years, and who has had considerable experience with finishes and their attendant problems, had nothing but rave reviews for this finest of lacquers.
USE OF LACQUER
Our FL lacquer is a very hard lacquer, and will polish to a high gloss. We've heard no complaints about cold-checking, but with such a hard lacquer this should be a consideration in colder climates (where if your instrument is exposed to extreme cold the finish will shrink but the wood won't, leaving you with a cracked surface). If you find it too hard for your purposes, 1 oz. of zinc stearate per gallon of lacquer should soften it adequately.
If all the preparation work has been completed (see Bill Wood's article on Fine Oil Finishing) and you are ready to finish your instrument we offer a few guidelines: Our thinner, FLT should be used. This thinner was formulated to be used with our lacquer; should you use other thinners you can expect drying and flowing problems, and you can expect rub-out time to increase, too. Use other thinners to clean the gun if they are cheaper.
There is much written on the subject of spray lacquering, and there are as many methods for finishing an instrument as there are instruments (well, almost). The following method has been used most successfully:
1. Wash coat all surfaces with a smooth, wet coat but not wet enough to run.(Thin lacquer approx. 25% lacquer to 75% thinner.) On resinous woods, such as Indian rosewood, you might want to spray on two light coats of shellac, no more than a two pound cut (2 pounds shellac to 1 gallon of alcohol).
2. Scuff-sand lightly with #320 paper to remove "hair."
3. Fill all porous wood with either our FT, transparent fill, by squeegeeing it on with stiff cardboard, or with a pore filler of your choice.
4. Sealcoat just as you did the washcoat. Color (our FS alcohol stains) may be added to the lacquer at this time, if desired. (The final few topcoats should be clear lacquer.) Remember to remove the shine from unfilled surfaces. Spray as many sealcoats as necessary until a uniform matte surface is achieved without sanding through to wood. The purpose at this point is to get the surface flat. This may take as few as two coats and as many as 10 coats depending on prior surface preparation. These can be sprayed at about one to two hour intervals with no more than four coats sprayed in a 24 hour period. (The solvent needs to be allowed time to thoroughly escape.)
5. Body the finish by spraying wet coats each hour in a warm, dry place. Find the mixture ratio that suits your gun the best. This depends on air pressure, temperature, and number of coats to be applied. (One of the men here sprays full strength, but he has an air compressor.) Sand the final coat or the one before the final coat with #600 wet/dry. Use a lubricant (soapy water or kerosene) and a rubber block. If you are spraying in an area of high humidity, you might encounter blushing (a milky haze) in the finish. You can relieve this problem by spraying on a light coat of straight thinner.
6. Finally, after one more day (preferably more) to dry, rub the finish out with our abrasive cut polish, FCP, and if you want a really glossy finish use our FFP fine polish. If you want a really-really, glossy-glossy finish, then follow the FFP with our FSP super fine polish/cleaner. If after polishing you find that your instrument needs to be resprayed you can remove the polish with dilute alcohol (with water).
USE OF THE SPRAY GUN
1. Check the gun for break-up. Hold it up between a light and yourself to see if the fluid is misting when it's a foot from the nozzle tip. If not, clean the tip with a wooden splinter or solvent or both. Screw the tip on very tightly and make sure that it seals. If it leaks behind the cap (tip) then remove it and clear the cap's seal at the end of the spinner. If it still won't break up, then the finish is probably too thick.
2. Spray in a smooth motion, overlapping 50% with the last stroke. Place a light so you can see the shine of the finish going on. It should be thick enough to run together but not heavy enough to run. If the gun is not held closely enough to the surface or if the air pressure is too high you will encounter overspray. The lacquer will dry on contact and will not be absorbed by the surface which will look rough and gritty. If you spray too closely, you will encounter sags, bubbles, orange peel and the like.
3. Always start the spray pattern with the gun pointed off the work, then bring it past the work smoothly, then back on, half a spray-width lower, etc., until you've covered the whole surface.
Text From The Luthiers' Mercantile Catalog - 1993
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