With your sealer coats already on, is the surface pitted or open pored in any way or does it resemble a flat,
totally sealed off surface? You will obviously have a better shot at it if there is a totally sealed smooth surface with no irregularities. ( probably stating the obvious)
Have you tried applying lighter coats, but with more time between each to allow the solvents to pop?
But maintaining your psi rating around 30 ? (It'll atomise the particles more evenly) Shooting at 15 psi wont improve anything, just give you more orange peel. Are you putting much heat into the body? using lamps, booth ?
If you need a hand, I'm only down the big road..................... in Melbourne.