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Old 05-27-2009, 04:53 AM
Laurent Brondel Laurent Brondel is offline
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Join Date: Jan 2009
Location: West Paris, ME
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To add to what Bruce and Howard wrote, my understanding of the term "varnish" is any finish that cures by polymerisation.

The classic shellac (or/and French polish) and nitrocellulose finishes cure by evaporation, meaning when the solvents are gone, what's left is the "finish".
The term "spirit varnish" can mean a lot of things, but mostly it is a shellac with added gums and resins (to modify elasticity, hardness and light refraction), so it cures by evaporation, but also polymerisation (it hardens more and more with time).
On a side note, very old shellac finishes tend to become impervious to their solvent (denatured alcohol), and very tough. Whereas nitro just disintegrates, perhaps slowly with the better products available now.

Oil varnishes cure entirely by polymerisation. I am not a chemist, but as I understand it, short molecules becomes long molecules when exposed with oxygen and/or UV. A short oil varnish has more resin per oil ratio, so it is tougher and harder. A long oil varnish has more oil per resin ratio, so it is more elastic and is suited for exterior work (most also have UV blockers). Woodwork on a boat or a deck tends to expand and contract quite a bit, and rapidly, a short oil varnish would just peel away after a short while.

IMO there is nothing that looks and feels better than an oil varnish, it probably has to do with how it refracts light on wood, a kind of kaleidoscopic experience. Also, as Bruce pointed out, it is pretty un-toxic as far as finishes go, and user-friendly if one is not in a hurry. It is virtually impossible to do a complete oil varnish on a guitar in less than 10 days.
It is also not too difficult to end up with a relatively thin finish (mine are always about .004"), which will not hamper the tone.
It is my opinion that whatever hard and elastic finish will not affect the tone negatively, as long as it is thin. I personally can not hear a difference between a thin French polish, nitro or oil varnish finish.
Most modern finishes tend to be around .008", sometimes .010", and it seems to me those guitars would sound much better with a thinner finish.
Also, a tung-oil based varnish thinned with turps smells wonderful…

I think it's kind of a misnomer to call modern polyurethane, urethane, catalysed or UV cured finishes "varnish". Even if they cure by polymerisation (and much faster than a short-oil varnish), the finish looks and feels nothing like an oil varnish.
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