Material science when it comes to finishes is endless and can be exhausting. Lacquer itself goes something like this: Originally there was Shellac (shell-lacquer), which was the crushed shell of a beetle mixed in a solution of alcohol. Then they synthesized it and came out with Nitrocellulose Lacquer (what they painted many 20s - 50s cars with). Even Acrylic Lacquer has been re-formulated using a much better modern Acrylic these days, and except for how it works has almost no resemblance to the early acrylic chemical-wise. The issue with Lacquer has always been it's porosity, meaning it has microscopic holes all the way through all layers. It "drys" not "cures" leaving microscopic holes where the solvent has evaporated from. Also it is very hard, durometer kind of hard, not toughness hard. (Think of glass as compared to rubber). This is the reason you can get such a fine finish as when you sand and rub you take the tops of the irregularities down to a very flat surface resulting in a high gloss. Moisture also transfers through it very easily. This is why when exposed to weather and temp extremes it doesn't hold up so well. Ideally that is going to be a much lesser issue on a wood instrument that isn't parked in the driveway in the snow, and then summer heat.....
"Enamel" has so many variations that it would be impossible to explain them all. There are literally hundreds of paint formulations that say "Enamel" on the can, and have no resemblance to each other. Just a couple to make the point are water-based, oil based, (think house paint), Synthetic and Acrylic, none of which are compatible formulations with the others. Oh, and catalyzed and non-catalyzed. With Enamel you get what you pay for. The stuff that comes in aerosol cans from Wall-mart is the cheapest crap of the bunch, never use this stuff on anything you care about. The best (in my opinion) is modern catalyzed Acrylic Enamel made for spraying cars and can be purchased at a real automotive paint store. It contains a catalyzing plastomer and when mixed with the hardener before spraying will set up as epoxy does. Very good stuff and tough as nails.
Modern catalyzed Acrylic Enamel can be finish sanded and buffed after a reasonable amount of time, I like a month. Be aware though that Enamel goes down in much thicker coats then say Lacquer, and if it is not a solid color, or clear, then you will ruin it by sanding and rubbing it because any metallic is "floated" in the layer when sprayed. You will just cut into the tops of the metallic by sanding and it will look really bad. The typical way to handle a solid color custom job in enamel (on a car) is to spray all the color coats, let it completely cure, cut and buff it, clean it properly, THEN spray a couple of wet coats of clear for the final gloss and leave it alone. All coats are mixed catalyzed. Enamels "cure" not "dry", meaning they set with heat, in our case that is the heat in the room and are way less porous.
BTW, Varathane used to mean Urethane Varnish, which contained no varnish, just colored urethane. I suspect that there is no urethane in Varathane today, only Polyurethane. Varathane is great stuff for its proper use. Don't confuse Urethane with Polyurethane though, not the same. Both Urethane and Polyurethane "cure" not "dry".
I realize that this can be really confusing. Thank the marketing departments......
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