RE: What is best 'House Paint' to use? And, anything special used for Airbrush thinning?
If you want the best gloss finish you probably need to get it from the final coats. And those would need to be clear. And the thicker the better, so you'll need more than one or two top coats. If you're needing to fuelproof latex, then you're probably going to need three clearcoats anyway to insure surface integrity. So if you're looking for gloss, you'll be getting it anyway.
BTW, if you're wanting fuelproofing and want it to be flat, with the old paints, you really needed more than three top coats because the flat was more porous.