I got this info from Bruce Sexauer:
Here are 2 versions of my methods I wrote out at different times:
Begin forwarded message:
I have been doing Varnish finishes since shortly after I started building guitars in 1967. After perhaps 25 guitars I moved toward the industry standard of Nitro-Cellulose finishes and worked in that medium predominately for 20 years or so. Then 10 years ago I realized I was having a bad reaction to the Nitro and recalled my fond memories of the friendly seeming varnish textures and smells, so I tried that once more. After a couple of years I dropped Nitro all together and am doing Oil Varnish as my standard finish.
I have used a number of different products, and get decent results with all of them, though some seem to smell better than others, and some are definitely harder than others, at least in the short term. I like the (763) 780-0964 German violin short oil varnishes, but they are soft for me. I have settled, for the time being, on a (763) 780-0964 product called "Rockhard Table Top" Varnish". The viscosity works for spraying right out of the can when fresh, but quickly thickens over time and requires adjustment. Remember how it started! After the first coat, thin with +/- 5% acetone, and enough Ace Hardware turpentine to get the "right" viscosity.
I use a Devilbis touch up gun with a standard tip, 1.0 whatever, I think. It is extremely important to sand the complete surface between coats. I use Norton Champagne PSA #320 (dry) on a medium block such as hard foam or felt.
I use Pore-o-pac oil based filler, and seal first with another Behlen product called "Violin Varnish", actually a shellac. It can be sprayed also.
I generally achieve a level surface in 5 coats, 1 a day. I level the fifth (sometimes the sixth or even the seventh) with Norton Abrasives #800 in water (saliva actually). I spray the last coat then a little wetter, or at least not dryer, than the others. I sand it with Norton Black Ice #1200 to absolutely level. After 3 or more days (2 weeks, if you have it) I buff with a pedestal machine and 12" wheels using first Minzerna Brown, then the other wheel with Minzerna creme. DO NOT buff the edges, it is much more easily burnt through than Nitro. Clean and do not wax.
2 years later:
Currently takes 6 to 8 coats.
Seal with Behlen violin varnish thinned 50/50 with alcohol
Pore filler I like is Pore-o-pac solvent based. I color it with MEK
dyes and artists oil paints.
Another coat of shellac after 18 hours or more.
I currently favor Ace hardware brand interior Oil Varnish (ACE
16389, and/or 276A111). The consistency is about right when you open
the can, but thin it with 10% Acetone. No more! As the can gets used
it thickens a bit, so by the end I add another 10% Turpentine. I
have found Ace Hardware to be the best brand for both solvents!
36 hours or more between coats is right.
I scuff sand with a hard block (pieces of flip-flop sandals are
extremely close to ideal) using 320 Norton Magnum PSA Don't try to
flatten it till the 3rd coat, pores remain for me until the 5th or
6th coat.
When the pores are gone I sand the next coat with 800 wet
one more coat , wait 3 days, then sanded with 1200, then buff.
Sanding the first coat is about 45 minutes, the last about 2 hours.
I use a cheap touch up gun to spray the varnish. After spraying I
put the varnish back in mixing jar and seal it back up. The Acetone
seems to keep it from skimming. If you can't smell the acetone, put
in a little more. It is pretty volatile and disappears over time. I
put a couple of ounces of Turmentine into the gun between coats,
sparay it a bit to get it up in there, and then leave it, pouring it
out before I load for the next coat. Need to actually clean the gun
every 3 months or so. I use lacquer thinner for that.
Bruce
2 more years:
Using Zinnser Sander sealer (a shellac) instead of behlen's stuff. Do NOT use their shellac, it has wax in it.
I am using foam backed sandpaper from Norton. #M makes something similar. Very useful for between coats sanding as it gets into the gullies. Not as useful for flattening. 320/400 early on, later 500/600. No use yet for the finer stuff. It’s called “Soft-touch". I get my abrasives from H and H Supply at: (763) 780-0964.
7/28/16
Te last few guitars have been sanded with 1200 wet on the penultimate caot, and 2000 wet on the final caot before buffing. I am currently waiting 3 weeks for the final coat to dry, though I doubt more than 5 days makes a real difference. Three guitars in a row completed at 6 coats!
Lacquer thinner now bothers me so much that I avoid it altogether at this point. The Harbor Freight touch up gun is $15, so when it gets too messy, I just replace it. One gun does between ten and fifteen guitars.
Bruce Sexauer
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- but I should be able to finish up the Challenge Build guitar with it.