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How many classical builders nitro the back and sides and French polish the top only vs. either FP or nitro entire box? Do you think it makes a difference in overall sound of guitar?

I would certainly favor FP on the top of a guitar (classical) as opposed to nitro. Even a very nice thin nitro finsh (.004-.005") will be twice the thickness of a good FP.
As for the importance of FP on the back and sides less so, but I think this is an overall question of aesthetics and if the guitar will be subjected to heavy usage in various conditions.
I FP my guitars completely because I like the look of this finish and I'm simply not interested in using lacquer in my home environment. There are some exceptions to this but I like being in total control of my own guitars at the finishing stage.
FP stands in contrast to thickly applied glossy finishes where the character of the wood underneath is obliterated.
There is however a limited audience for this type of instrument.
Players who are more interested in sound and playability than a nice "shiny guitar"
appreciate them because they understand that the thin finish is adding to what their ears are hearing.
This of course is all dependant on their ability to appreciate the difference, which alas, many do not possess.

At this point I'm focusing not on changing my instruments to please players but to seek out those who can appreciate them for what they are and simply weed out those potential clients who can't.

No argument there Todd and I think it would be irresponsible for those of us who prefer FP to advocate it for all uses.
Your typical Bluegrass Dreadnought player on the hot and sticky summer festival circuit is not going to be served by a guitar with a FP finish. I also don't see much of an advantage to FP on the much heavier built steel string guitar unless one is shooting for a 19th or early 20th century aesthetic.
Classicals tend to be used in more controlled situations and walk a very fine line between structural integrity and maximum responsiveness and thus FP I think is a much more viable finish there.
Most importantly though is the marketing aspect of FP in that it is still considered the ultimate finish for sound quality by the classical guitar buying public and thus overall is an advantage (in most cases) as it is widely accepted in that genre. I think getting the guitar buying public to accept the look would be an uphill battle in the steel string market though.
Hats off to those who are keeping their lacquer finishes as thin as you have described, the tendancy to go thick (poly and UV cure "factory" finishes) for that featureless "glassy" look puts us all in a bad position I think as it sets a wrong headed standard for hand built guitars.
Best

There's certainly an attitude difference. Modern french polished Classicals are positively over finished compared to early 19 th century Guitars. I know that those Guitars were largely played without Nails (certainly not all) but there are numerous examples that are in remarkable condition some 200 years later. I also think that Guitarists who are playing original instruments simply aren't concerned with a slightly grubby soundboard or wear patterns to the Back/Sides. More in the manner of Violin players.

Do you mean Michael that they truly regard the guitar as a musical tool as opposed to a piece of shiny furniture which serves to reflect their ego??

.........oh dear.......