The question I ask is why would you want to. You always stand a chance of having a nasty chemical reaction between the two that will cause you unnecessary grief. What if the topcoat dries sticky? IMHO I would pick a good horse and ride it. (:-D)
If you're using oil-based paint you'll need oil-based lacquer. If you want to seal it first, shellac will seal almost anything. I don't see the point in doing that, though. I'd just use the oil-based lacquer.
Shellac uses alcohol as a solvent--which doesn't really affect either oil-based or water-based products. When in doubt put a "sealer coat" of shellac between layers to prevent them from hurting each other.
If you're buying pre-mixed shellac, it's best to use the de-waxed variety. In the US, Zinsser brand "Seal Coat" shellac is about the only de-waxed available. All other Zinsser products still have wax in them. The wax might keep your top coat from adhering properly.
I love shellac. Anything that I build that won't be used for a table or something that gets a lot of wear I finish with shellac. I also use it to seal wood and pore fill. It's pretty much the most versatile and easiest finish to use.
I believe cabinet guys mix a dilute coat of shellac that they call a "spit coat" to toughen up the wood (especially the end grain) before applying any finish.
Oil based Lacquer. AKA Clear Enamel.
This is why I ask: http://www.cigarboxnation.com/photo/tobacco-burnst?xg_source=activity It is already painted. Actually it is artists paint. I know this is not the best choice.
Do you think it is strong enough?
google for "varnish over artist oil paint" and read up on which varnishes work well over what types of paint. Many of the "great paintings" are varnished after the artwork is done to protect the paint from dirt/oil/dust/humidity/etc. and your CBG Mona Lisa is worth no less love and care.
Thanks I've checked that, but I am afraid those varnishes are for paintings on the wall not a guitar in the hand. Do you think they are strong enough can take the sweat of your hand?
hmm, 1st coat a proper and type appropriate varnish for the paint being protected, 2nd-Nth coat a good spar varnish (made for boats) ought to stand up to the sweatiest picker at the summer solstice barn dance.