I've tried a lot of different finishes over the years, and there are few of rules of thumb that come to mind.
1) They all take about the same amount of time from start to finish.
2) The most important step, by far, is the surface prep.
3) There is no 'perfect' finish. They all have drawbacks and advantages, and all you can do is find the one that comes close to meeting your requirements without having any faults that you can't tolerate.
Nitro is the 'standard' for a reason, but it's not a reason that might sway you as an individual maker. If you have a good spray rig and the requisite skill it goes on nicely, dries reliably and hard, and looks terrific when buffed up. Touch ups and new coats 'burn in' perfectly. Now for the drawbacks.
Nitro is, itself, the 'little brother to a high explosive', and the solvents used are very flammable. Of course, any flammable substance when finely divided in air can explode, so this stuff is a first-class fire hazard. The requirements for a _legal_ spray setup are formidable, and expensive, at least around here. Yeah, you can get away with spraying it outdoors, but that's not nice either. All of the solvents used are toxic in concentrations you can't even smell, for one thing; there goes your 'green' rating. The nitrated part of the cellulose molecules that make the stuff soluble at all is chemically unstable. That's why nitro turns orange, shrinks, and cracks with age. The nitrate fumes can etch plated parts, too. I used to use nitro; now I can hardly open a can without getting a headache.
Shellac has some of the advantages of nitro, in that, as a solvent release finish, it 'burns in' perfectly. It's actually a fairly hard finish, and more resistant to chipping than nitro. Shellac also gets better, harder and more solvent resistant, with age, although it takes about 75 years for the process to run it's course. You can brush shellac if you want to thicker build-up, although its a bit tricky to get a really level finish that way. A few drops of kerosene or spike oil (kero is a lot cheaper) in a pint of shellac make the brushing go better. I've heard of people spraying shellac by adding some acetone, but I've never tried it.
Shellac is soluble in alcohol or alkaline water solutions. This is both an advantage and a drawback. Alcohol is not nearly as poisonous as lacquer thinner, although some of the stuff they put in to denature it is pretty bad. You can get 190 proof grain alky in some states, and that won't be as bad for your health, although your wallet might object to paying $15-20 a quart for shellac thinner. Put a little in your coffee and the problem seems less pressing, though.
The drawback of the solvents used with shellac is that they're a lot more common than, say, the toluene or MEK used with nitro. For many people sweat is an alkaline water solution, or so it seems. Also, since alcohol is less volatile than lacquer thinner, it takes a given thickness of shellac longer to fully harden than the same thickness of lacquer. Shellac also has a shelf live, both dried and mixed, and it can be drastically slowed in hardening if the humidity is high.
French polish is a method of applying shellac (or other resins) with a pad, to get the thinnest possible coat. Because there's less of it, your'e adding less weight, and that can help the sound, particularly on something like a classical guitar top. There are a few drawbacks, of course. You can't hide ANYTHING under .002" of clear finish; your surface prep has to be perfect. That thin coating cant protect the wood much against scratches or dents; there are few things as fragile as a cedar top with a new French polish finish. Sincemost players these days are used to industrial type finishes that wear relatively like iron, it's a given that taking a guitar with an FP'd top to a show guarantees a harvest of dings. Finally, an FP job always looks a little better the longer you spend on it. Thus the term 'French polish finish' is an oxymoron; once you start you'll never really finish. That siad, you can do an adequate 'quickie' job with FP faster than almost any other finish, so it has the shortest lead time.
'Varnish' is really a whole class of things, traditionally made by cooking a drying oil and a resin together. There are at least a half dozen different oils, and a couple of dozen resins, that can be used, and the product will vary depending on the ingredients, the proportions, the cooking time and temperature, and probably a bunch of other things as well.
Recently I've been using Behlen's 'Rockhard Table Top' varnish. The first coat can take as much as a week to fully cure over some woods, but once it has subsequent coats will dry in less than four hours. UV light helps that first coat to set. I thin it with about 25% of their proprietary reducer, 5% acetone, and a little kerosene as a brushing agent, and apply 5-10 thin coats. Make sure you do the last two coats the same day, and allow it to cure for t least a couple of weeks before leveling and polishing, to avoid witness lines. This goes on thicker than FP, and it's a pretty tough finish as well, so it's much better protection.
True Oil is essentially a thin varnish that you wipe on. In that sense it'slike FP; you can just keep putting it on and it keeps looking better. You can do a minimal coat quickly, and, of course, get less protection.
All of the varnishes I've tried keep shrinking for at least a couple of months after they're applied, so if you want that perfectly level 'Lexus look' you've got a bit of a wait. I like the look of wood grain, myself. Any varnish will harden in the can if there's air in there. I add marbles as I go; some folks swear by 'Bloxygen'. The only way to get all of the varnish outof the brush is to wash it well with soap and water. I spend about as long cleaning the brush as I do actually putting on varnish. Lately I've been using a 1/2" camel hair brush, and am on the lookout for a 1" one; it gives a nice smooth coat.
Most of the polyurethanes I've tried seem to stick better to themselves than they do to the wood. They tend to peel over time like a bad sunburn.
All the experinience I've had personally with water base finishes was several years ago, and it was all bad. One of my students has used a KTM product, and got good results, but it was very hard stuff to polish out.
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Once digested I can make a more educated decision.
so a satin finish may be the way to go to hide it?