One bummer about Rustoleum is that it doesn't tolerate some auto fluids, for example brake fluid. It's cheap, the colors between batches match well, it's easy to work with and I use it quite a bit though on the car I usually use a cheap single stage acrylic. I buy Rustoleum in quart or gallon and thin it with mineral spirits and use an HVLP gun. Better finish than brushing and way cheaper than spray cans. Though if I'm too lazy to clean the gun and the job is small I'll go get a can.
The POR 15 is a great idea though it's not cheap. My donor is a Kansas car and rustier than I thought at inspection. Once the interior came out it was obvious that the top had been damaged for years. I just ordered another couple of 10 amp angle grinders. I had three, one just died, one is about to die. I like to have knotted wire, grinder and flap or cut off wheel handy without changing. The floor isn't structurally compromised so I think I can get most of it off with some work. If it's too bad I can do it with a quart of POR 15.
for you guys concerned with weight, I take both the interior and the body down to the metal and reapply, no primer. It's a race car. It's going to get bent up before it rusts out.
Dave