Finishing Your Instrument With Tru Gun Oil
By Chuck DeHart, Fulks Run, Virginia 7/11/96
This article was originally written for posting to the newsgroup: rec.music.makers.builders.
Some recent discussion and a handful of requests moves me to relate the guitar finishing technique in which I was trained. I am not affiliated with Birchwood-Casey, nor are my teachers Anthony Huvard and Bill Davis, Jr. (Northwest School of Instrument Design, ca. 1980, Seattle and Skykomish, Washington). As I expect with this newsgroup, others are encouraged to add and comment because I do not know it all. I have finished six instruments of my own construction with this process. I have finished one instrument in lacquer. I have also been a regular reader of Fine Woodworking where I would recommend others search for more information on finishing wood products.
Safety first!
Remember that finishing rags can spontaneously combust and ruin your day/life. Dispose of them properly (in a bucket of water? Who can help me here?) Most finishing products are flammable and poisonous so use according to directions.
I think the best way to talk about this finishing process is to: 1) talk a little about some of the differences between Tru Gun Oil and Lacquer, 2) highlight some important aspects of final sanding, the shellac washcoat and filling pores, and 3) discuss in some detail the varnish finish.
Did I say varnish? Yes, Tru Gun Oil is actually a varnish. See Fine Woodworking (sorry, don't remember the exact edition) within the last year or two for an article about the difference between an oil and a varnish. (For example is linseed oil an oil? Is boiled linseed oil an oil? How about tung oil?)
Tru Gun Oil vs. Lacquer
I was taught to especially use a varnish on classical guitars. It is softer and less likely to effect sound. Likewise, it is not as durable as a hard "plastic coating." The varnish finish may dent and scratch easier. It does feel softer and I like that when I play my guitars.
A varnish may be applied with more tolerance to temperature and humidity. It is more easily applied by a brush or soft cloth, no spray set up is necessary. A varnish thins with turpentine (any brand will do) while I understand that the lacquer thinner should match that recommended by the manufacturer of the lacquer.
Lacquer is a "hot" finish, i.e., each layer of lacquer chemically bonds to the layer underneath, becoming "one." Varnish does not! This may be the single most important difference to remember because it will drive other important processes of the finish.
The biggest "trade-off" for me between lacquer and varnish seems to be that you have less worry about your environment (temp and humidity), but must be more skillful because the finish is not "hot." For me, its been easier to build the skill than to try to control the environment.
Comments About Final Sanding:
The quality of your finishing job will be driven by the quality of the surface to which it is applied. Final sanding is critical and if you spend a lot of time on it you will save time later. Number one, go for an even finish, no matter what the final grade of sandpaper used. I sand to the backside of 600 grit. Make sure that there are no high spots (like the end grain of your custom rosette) or low spots. When you sand between coats later, you will probably sand through these high spots. In low spots a "pool" of varnish may collect and not fully harden.
Comments About The Shellac Washcoat:
I apply two "washcoats" of thinned shellac with a light sanding between coats. Pay special attention to rosewoods and other bleeders (one reason to finish the bridge and pickguard separately). A rosewood binding will bleed onto your spruce top if you are not careful. You can diminish the effect of bleeding bindings by VERY CAREFULLY "washing" out the oils with denatured alcohol (the same solvent for shellac). (On my cutaway, I made a three piece neck with book matched curly maple sandwiching a piece of coca bola. I allowed the bleeding to mix onto the maple, using denatured alcohol to wash it out and add a very slight red tint to the maple.) (Although I have never used it, I believe that pure grain alcohol is the same as denatured alcohol without poison in it. Has anybody tried using it? I would want to avoid the poison and am willing to pay the liquor tax.)
Comments About Filling Pores:
If you don't, the pores will suck down your finish and you will see these dimples later. Your sandpaper will not hit them and these low spots will have a different gloss than the surface. Also, if you add oil colors to the filler, consider using Japan Dryer to compensate for the oil added (to help harden the filler because you changed its chemistry when you added oil). A little goes a long way.
Do a third washcoat of shellac!
The Tru Gun Oil Finish:
If you change your mind about Tru Oil or any other varnish at this point in the process, you can still do a lacquer finish. You have laid the base for a glossy finish for many products.
Your first temptation to achieve a high gloss finish will be to try to get a couple of quick thick coats applied. Resist this temptation. Remember that one layer will set on top of the other. If you develop a drip, you will sand through it between coats (400 grit wet). The varnish inside the drip will not harden. As you sand away this high spot, you will create a ring where the high spot meets flat (good finish). The more you attempt to sand away this tiny "ledge" of finish, the bigger this ring will get. You cannot escape this vicious cycle and you may sand through the layer underneath also. Each time you sand to the surface, you are starting over again on that part of the instrument (thus the 8 to 10 coats routine).
I started using a brush and now use a soft, lint free cloth to apply a thin coat (full strength from the bottle). The trick of experience is to know how thick you can make the coat without creating a drip. Large flat surfaces, lke the top and back, handle well. Watch especially for drips at the edges (bindings), the lower bout (if you hang the instrument from the head) and detail like the machine wells (classical guitars). Although your shellac washcoat tacked almost immediately, you have a little more time to play with wiping away excess varnish, but do be careful.
Light wet sand between coats, 400 grit sand paper. Especially light on the first coat. Remember that the main purpose for light sanding between coats is to remove slight undulations in the finish left from the brush, cloth or flecks of dust. If you final sanded properly, you should not sand through the finish because there are no high spots. Use the same sanding block, making sure that it is not one of the plastic ones you buy in the hardware store because they tend to flex and not hold a true flat surface. Look for the tiny flecks of dust that stuck to your finish because your environment is not perfect. You may want to use a thumb sized piece of sandpaper on these tiny spots. (Use a tack cloth before applying each coat.) Watch out for sanding through the finish at the bindings or other edges. Varnish does not stick well to oily woods, like rosewood. If you sand through your washcoat, and into oily wood, you may have to reapply the shellac washcoat on these small spots.
Maintain good lighting from several angles so you can see what is happening to your surface. Clip lamps, lots of clip lamps. They also can help with drying if humidity is high or temperature low.
When you think you have the entire surface area coated, lightly sand to the backside of 600 grit sandpaper and check again. Sandthrough will usually show a lighter color, especially after your wet surface dries.
To polish the finish, mix rotten stone with mineral spirits. (FFFF is, I believe, actually tin oxide.) Make a paste and rub it onto the surface with a soft cloth in circles. Spirit off any accumulation with mineral spirits and check for sandthrough again. Again, sandthrough will be hard to miss after the mineral spirits evaporate and leave rotten stone dust in your grain.
Finally, I use a floor wax, Johnson's, to add the final sheen and protection to the finish. Apply with a softcloth per directions. (I don't work for Johnson's either).
Having said the above, would anyone care to talk about French polishing? Its a process I would like to know more about.
Final Words: Jam on and don't worry about keeping your finish perfect!