woops, i read the original post wrong.
i thought you siad that you wanted to airbrush a clear coat on, but now i just realized that you want to put a clear coat over airbrushed paint.
in that case, still use devcon, but dont thin it at much.
Big M is right, thinning weakens it. unthinned devcon is the strongest possible.
i still like to thin mine some though, because thinning it makes it SO MUCH easier to brush on. i find it very difficult to evenly brush on unthinned devcon. i also find that thinned devcon gives a shinier coat.