Rattle can primer

I agree get some sort of inexpensive water trap , also you can also get a $5 little plastic bulb type trap to use at your gun, it helps. The foundation of your job will be the quality of the metal surface and using a good epoxy primer. I agree stay away from the auto parts store crap. Only thing I have seen decent is a large can made by MarHyde and I think they called it a self etch. I have used it as a spot primer on a few small spots with OK results. The stuff is expensive like $35 for a large spray can. BUT!!! Don't try to prime your car with stuff out of a spray can!!!! Period. OK?
Like many others on here, I like Southern Polyurethanes stuff. You can get an epoxy primer that makes 2 gal with the activator. Also you an Mix a little reducer and use it as a sealer over your filler, you applied on top of the epoxy primer. So you spend maybe $250 for this.
So for that $ you have enough epoxy to primer the whole car very well top bottom inside out and seal it with great quality product.