There was a large post at the FMM bulletin board recently regarding finishes used on the 'ukulele. It got quite heated as various luthiers weighed in with their opinions on the best type of finish for their particular situation. I found it kind of amusing and very informative. Between my limited personal experience and what I've learned from threads like this, I think I have a pretty decent idea about the pros and cons of each type of finish. Here is a summary of what I've learned:
French Polish: This yields the thinnest finish on the uke and takes the longest time and most skill to apply. The thin finish allows for the best sound possible and it looks gorgeous when done right. It is also the easiest finish to repair, assuming the luthier working on it is good at French Polishing. The problem is that it is also very fragile so great care needs to be taken on such an instrument. The Glyph Mezzo-soprano I have on order will have a French Polished finish. I am having a clear pick guard put on it as I know I tend to put scratches on the upper bout.
Lacquer: This yields a thin finish and can be done with a high gloss look. It is pretty durable but does show aging and supposedly lasts about 70-80 years or so before needing to be re-finished. I guess that's not a big concern for me as it should outlast me anyway and the re-finish would be the next owner's problem.:P It's supposed to be moderately easy to repair, although some thinks that it's hard to do an undetectable job on a repair for this finish. It does seem that lacquer finish isn't so good for the person doing the spraying as it is pretty toxic. My Kepasa Little Mac will have this type of finish. I'm not too worried about scratching it because it will have a cut-away, but it's supposed to be fairly durable against scratching anyway.
Poly: This appears to yield a thicker finish than lacquer although some luthiers say it can be applied just as thin as the lacquer type finishes. Most of the manufacturers use this type of finish as it is fast and more cost effective in a production setting. I think just about all of my ukes have a poly finish. On the KoAloha Pineapple Sunday and G-String soprano, the finish seem to be pretty thin and the instrument vibrates very freely as I strum. The Honu and Pono, with a glossy finish, seem to have a thicker finish. The Honu does vibrate very well. The Pono does seem a little more restricted but nonetheless it is loud. Poly finishes are very durable and supposed to last hundreds of years. Some believe the non-biodegradability makes poly finish not very environmental.
UV cured Poly: This is something that Kanile'a started using in 2006, which they learned from Taylor Guitars. It yields a very durable finish and makes the production process faster and more efficient. The UV chamber cures the finish much faster than normal poly finishes that has to cure in air. My Kanile'a super-soprano has a very high gloss mirror like finish which some likens to dipping the uke in plastic. I think it looks great but it does look thicker than other finishes and the uke doesn't seem to vibrate as freely as the other Hawaiian made ukes I have. Still, it sounds great and I have no problems with its sound.
So, that's what I gathered so far in my learning about uke finishes. There are also oil type finishes but I'm not real sure about their strength and weaknesses other than reading that they need to be re-done every year or something like that. I think for me personally, laquer and poly finishes probably work the best because I seem to make contact with the upper bout pretty regularly while playing. While I don't strive for perfect looking instruments, I do like to keep them looking nice instead of trashed, so a durable finish is fairly important to me. I'm hoping the clear pick guard will do the trick for that Glyph I have on order, as French Polish doesn't seem to suit my playing style too much. But I do understand the pros and cons of each finish and feel like I know enough to deal with each type of finish.