Jack, there are catalyzed poly finishes that are near impossible to solvent dissolve after being cured.
Fender uses a finish like this on all their insturments, except the really low end imports, and the USA reissues (and a few other models with nitro finishes). The low end imports have such bad wood, they use polyethylene as a finish, and it is a structural element in the guitar!~
On a Fender finish, you can nitro over the "poly" all you want. Its just about impervious to everything except very very strong solvents. Many paint strippers wont even touch it.
What Warmoth uses... I dunno. I know Frank Ford uses his "nitro test" where he puts a small dab of acetone on a concealed area to see if it swells or damages the finish. Most guitar finishes that are not nitro, fully pass the test and be nitro'd over. In fact, I've never heard of a factory finish that was "poly" that you cant nitro over, but its always safe to test.... and easy to do.
FOR DECALS - the safest way to nitro over a decal, is to use shellac as an intermediate coat. Shellac has the ability to stick to even polished metal, and will make a barrier between the decal and the nitro, sticking to both. Two coats of clear shellac fully dried, then nitro over that. Shellac also has the property of removing water from the decal itself, since its alcohol based. The water will cloud the shellac briefly (like it also does in lacquer) but this will go away (which doesn't always happen with lacquer). Thats a good feature!
ccasion14: 
