Refinishing Acoustic Instruments
Moderator: jingle_jangle
Refinishing Acoustic Instruments
Paul: Amongst my collection of used instruments is a 1967 Gibson J50 that has finish cracks galore. My question is, what special considerations are there when refinishing an acoustic instrument. Would newly applied lacquers, for example, change the tonality?
Life, as with music, often requires one to let go of the melody and listen to the rhythm
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jingle_jangle
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By "finish cracks", I assume you mean the usual spiderwebbing and checking that come from the old lacquer turning brittle and failing to expand with the wood during repeated environmental variations. This is very common with old nitro finishes.
Special considerations when refinishing:
1. Stripping
The nitrocellulose finishes on these are thin and easy to strip, whether by abrasives or with chemical strippers. Scraping should not be used.
Mask off the binding and rosette with good quality masking tape burnished well into place. Use a non-colored crepe paper masking tape--colored tapes such as the 3M Green or Blue, will bleed color into the spruce and mahogany or rosewood. Don't use bargain-priced tapes; their adhesives are inferior to that found on good 3M tape.
When you mask the binding, apply the tape to overlap the wood itself about 1/8", to allow for seepage, and apply three layers of tape.
Now put on your rubber gloves and brush on that thin coat of chemical stripper. Most of the hardware-store brands use methylene chloride as their active ingredient, along with a thickening agent and dissolved paraffin wax. Methylene Chloride evaporates in a flash, and I believe that the wax acts as an inhibitor to this (Gary? Comments?). There are also many brands of so-called "green" strippers available, though none I'm aware of is actually recommended for musical instrument work.
After brushing on the remover in a thin coat, you will notice the old lacquer finish begin to lift virtually immediately. Wait until the entire finish has crackled and lifted before beginning the next step.
On clear nitro, scrubbing with #1 steel wool that is dipped into denatured alcohol, works very quickly to remove the nitro. Wipe with a cloth--in this case, the rougher the better, and cheesecloth is the best.
Once the nitro is off completely (and this could take a second or even third application of remover/scrub cycle), a lighter scrubbing with alcohol alone will remove the last of the film of stripper/lacquer emulsion. Wipe or blow the instrument dry and let it sit for an hour or two before removing the masking from the bound areas.
Now you can carefully sand, to remove the nitro from the binding and adjacent areas which you masked off. Quality spruce is quite soft, so use #220 garnet paper on a hard rubber block, and try not to round off the edges and corners! The entire guitar is sanded next, and stay with the grain in this step. For the sharp inside corners of the heel to body joint, instead of using the block fold the paper over several times and use the edge you've created to work into the corner. Try to finish sand again with the grain.
Next: Finishing
Special considerations when refinishing:
1. Stripping
The nitrocellulose finishes on these are thin and easy to strip, whether by abrasives or with chemical strippers. Scraping should not be used.
Mask off the binding and rosette with good quality masking tape burnished well into place. Use a non-colored crepe paper masking tape--colored tapes such as the 3M Green or Blue, will bleed color into the spruce and mahogany or rosewood. Don't use bargain-priced tapes; their adhesives are inferior to that found on good 3M tape.
When you mask the binding, apply the tape to overlap the wood itself about 1/8", to allow for seepage, and apply three layers of tape.
Now put on your rubber gloves and brush on that thin coat of chemical stripper. Most of the hardware-store brands use methylene chloride as their active ingredient, along with a thickening agent and dissolved paraffin wax. Methylene Chloride evaporates in a flash, and I believe that the wax acts as an inhibitor to this (Gary? Comments?). There are also many brands of so-called "green" strippers available, though none I'm aware of is actually recommended for musical instrument work.
After brushing on the remover in a thin coat, you will notice the old lacquer finish begin to lift virtually immediately. Wait until the entire finish has crackled and lifted before beginning the next step.
On clear nitro, scrubbing with #1 steel wool that is dipped into denatured alcohol, works very quickly to remove the nitro. Wipe with a cloth--in this case, the rougher the better, and cheesecloth is the best.
Once the nitro is off completely (and this could take a second or even third application of remover/scrub cycle), a lighter scrubbing with alcohol alone will remove the last of the film of stripper/lacquer emulsion. Wipe or blow the instrument dry and let it sit for an hour or two before removing the masking from the bound areas.
Now you can carefully sand, to remove the nitro from the binding and adjacent areas which you masked off. Quality spruce is quite soft, so use #220 garnet paper on a hard rubber block, and try not to round off the edges and corners! The entire guitar is sanded next, and stay with the grain in this step. For the sharp inside corners of the heel to body joint, instead of using the block fold the paper over several times and use the edge you've created to work into the corner. Try to finish sand again with the grain.
Next: Finishing
“I say in speeches that a plausible mission of artists is to make people appreciate being alive at least a little bit. I am then asked if I know of any artists who pulled that off. I reply, 'The Beatles did.”
― Kurt Vonnegut
― Kurt Vonnegut
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jingle_jangle
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2. Finishing
First examine the wood under strong light and correct any sanding scratches that you may see which run across the grain, by sanding with the grain using a block if possible and sanding until they are gone.
Next step is sealing. You can use vinyl sanding sealer, shellac thinned 100% with denatured alcohol, or nitro thinned 50% with lacquer thinner. I recommend the latter. Here for best results, a spray gun is necessary. It puts on even coats which are thinner and dry consistently. Mask off the fretboard. The binding will dry last, as it is non-porous for all practical purposes.
Following sealing, a light sanding with #220 paper again (to knock down the grain which is raised by the sealer) leaves the surface ready to spray the finish coats. I would spray one double wet coat of nitro and let the guitar dry, hung by a hook through a tuner hole, for at least a day. Reinstall tuners and re-string.
Does a nitro refinish change the tone? It should brighten the tone a bit--depends upon how good your ears are, as it will stiffen the top plate somewhat.
First examine the wood under strong light and correct any sanding scratches that you may see which run across the grain, by sanding with the grain using a block if possible and sanding until they are gone.
Next step is sealing. You can use vinyl sanding sealer, shellac thinned 100% with denatured alcohol, or nitro thinned 50% with lacquer thinner. I recommend the latter. Here for best results, a spray gun is necessary. It puts on even coats which are thinner and dry consistently. Mask off the fretboard. The binding will dry last, as it is non-porous for all practical purposes.
Following sealing, a light sanding with #220 paper again (to knock down the grain which is raised by the sealer) leaves the surface ready to spray the finish coats. I would spray one double wet coat of nitro and let the guitar dry, hung by a hook through a tuner hole, for at least a day. Reinstall tuners and re-string.
Does a nitro refinish change the tone? It should brighten the tone a bit--depends upon how good your ears are, as it will stiffen the top plate somewhat.
“I say in speeches that a plausible mission of artists is to make people appreciate being alive at least a little bit. I am then asked if I know of any artists who pulled that off. I reply, 'The Beatles did.”
― Kurt Vonnegut
― Kurt Vonnegut