I have had great results from Future floor polish now marketed by Pledge, (Johnson's Klear in Europe) this is a clear acrylic.  I would experiment with a couple of old models first,  It can be sprayed with a airbrush,  brushed on, or added to  a paint pot to "gloss"  a regular color,  TRY them out individually before you commit to your project and see whats best for you.  Any clear coat in my experience tend to be temperature and moisture sensitive.  Future is self leveling but get in the habit of applying it and leaving it the heck alone for a while,  fingerprints in "tacky" future can blemish a otherwise good finish, you can sometimes fix it with another application.  One advantage of Future is it is water soluble for clean up.  I tend to airbrush future on a military model before decals and pin washes as the smooth surface helps eliminate air bubbles under decals.