I have a couple instruments with traditional French polish and, they looked great for about a week. Very quickly the finish turned to goo where my arm and hands touch and most of the finish a real mess after a few years. Look like it was left out in the rain for a few days! Of course I live in hot and humid Nuuanu Valley and I'm sure French polish will fare better in more temperate climates. On the other hand my instruments with polyester finishes look and feel like new after years of gigs and practice. I do think the French polish sounds better but proved not to be a practical finish for me.
Peter,
Your experience with FP must be very disappointing, although not that surprising, as it is well documented that some body chemistries react poorly with FPed finishes. The fact that it started to deteriorate after only a week seems atypical of a well executed job.
FPed shellac is one of the first finishes that I went close to mastering (perhaps not close enough), and I have one classical guitar build that I see fairly frequently that is still in good shape, finish-wise, after circa 25 years. It is part of a large collection, so it probably doesn't get a regular 'flogging'.
The downfalls of good FP application include: Using too much lubricant/oil, too many 'coats' in too shorter time with insufficient drying time between, coating too thickly, having too much water content in the alcohol solvent, and using old, age-affected shellac flakes. I can vouch for the last one after an elderly cousin gave me some flakes out of the back of his shed that he had last used on a school woodworking project. The flakes swelled and then turned to jelly without dissolving completely.
All of the mistakes listed have, sadly, been learned by experience, or more accurately, by inexperience.
I would never discourage any new builder from trying this process, as there is something about an FPed finish that soothes the senses.... perhaps it comes from the knowing that this is one of the techniques used by the old masters.