Ovation has found the secret of a perfect level finish on the top: a full millimeter thickness of epoxy, sprayed on in a single coat. It's level and stays that way until it crazes. What all that mass and stiffness do to the sound is not wonderful, however. I've replaced tops on Ovations, and they always sound much better.
This is one place where tone certainly conflicts with the 'flawless' esthetic. Finish adds mass and stiffness, and very often, damping to the system, all of which reduce some aspects of tone. You need a finish to protect the wood against dirt and moisture, but for that anything that forms a reasonably hard water and chemical resistant film will do. The 'perfect' finish in that respect might be a one molecule thick layer of diamond. That would not provide much mechanical protection against dings and scratches, so you do have to build up some thickness. In terms of tone, though, less is generally better.
Finish also contributes to the esthetic by providing a smooth surface and by bringing out the 'light' of the wood. Part of the look of the surface comes from having a finish with the relatively high refractive index, so that light reflects off it. On the other hand, a finish that has the same refractive index as cellulose will give a greater 'depth' and bring out the color and grain of the wood. The higher index finishes have a more of a 'surface' look. It's probably impossible to have both.
As far as I know, all finishes do tend to shrink a bit after they're applied. Normally they do most of that fairly quickly, but how quick 'quick' is varies. Epoxy, and possibly UV cure poly (with which I have scant experience), shrink very little. A solvent release finish, such as shellac or nitrocellulose lacquer, will take some time to 'outgas', and shrink as it does. Shellac takes longer to shrink back because alcohol is less volatile than lacquer thinner. A reactive finish like oil-resin varnish, can take quite a while to fully shrink back, since it first has to release all of it's 'vehicle', and then cure, often by oxidation. This can take months.
So, if a level coating with a bright surface is your goal, epoxy or UV polyester with a high index would fill the bill. You'd apply it fairly thickly, to minimize the effects of future wood movement. If tone is the objective, you'd do what many Classical guitar makers still do and French polish with shellac. It's possible to produce a fairly level surface that way even with a thin coating, but id doesn't provide much protection to the wood and can break down rapidly when exposed to sweat. Since it takes months to really stop shrinking you will see the structure of the wood, and things like glue lines, through the finish unless the maker holds it for long enough to work those out. In terms of bringing out the beauty and 'light' of wood it's hard to beat an oil-resin varnish, but those shrink even more and for longer than shellac. A good varnish will be tougher (more scratch resistant) than either nitro or shellac at a given thickness, but also more flexible, and thus prone to denting. And so on.
The point of all this is that all of the finishes have advantages and drawbacks. Many of the esthetic issues can be overcome by the simple expenditure of time and effort. It's entirely possible to spend more time on the finish than on actually making the guitar.
There is an old saying: "Everybody wants things that are good, fast, and cheap. You can have any two."
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