I'm not one hundred percent sure, but the french polish is much thiner than normal polish. The guitar top is more free to vibrate. But the counterpart is that french polish offers less protection to the guitar.
It's about 100% more work. That is why it's not used too much. All you want is a thin layer. Frenching is a good way to not put too much on and it looks really nice. The process takes a long time and if you don't know what your doing it can look like crap. Another reason that french might not be as good is that it is a bit more delicate. It does not stand up to bumps and scratching as well. Since it is shellack it can crack and will break down in the heat. Tends to get sticky where the guitar meats your body while playing. But it is easy to fix. So there is a give-and-take as with many things. Many higher end classical guitars use French.
This technique was firt applied to violins or cellos, and later on high end guitars (classical most of the time, but some jazz archtops as well). Excellent result concerning sound, but only experienced luthiers can do it right. Known as "vernis au tampon" or "vernis à l'alcool", extremely delicate, a light fingernail touch can remove the thin coating.
An alternative (but expensive because it takes a lot of time) : 5 to 10 layers of lacquer, with sandpaper work at each step, which results in a very thin coating, good protection, perfect gloss.
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Cheers,
Josh
An alternative (but expensive because it takes a lot of time) : 5 to 10 layers of lacquer, with sandpaper work at each step, which results in a very thin coating, good protection, perfect gloss.
Oops, sorry. Wrong forum.