I've got a pdf of a Model Airplane News article on painting with latex by Roy Vaillancourt, in which he says: "If, however, you are using a glow engine, you’ll need to protect the finish with a clear epoxy or polyurethane top coat after it has dried fully."
I'd probably use System 3 urethane clear, which can be had either from System 3 or from Nelson's Hobby. The stuff is enormously expensive, but it looks like you don't need much of it. There's also automotive urethanes, which, incidentally, are also enormously expensive. Sigh.
Note that the System 3 stuff is only fuel proof if you use the cross-linker. If you air dry it then it's not guaranteed.