I also prefer POR after a good sandblast or heavy scrape. It is a moisture-cure urethane and so helps prevent further rust even if a tiny layer of rust is present. It cannot do this rust prevention if it goes over old paint, though it will still stick. It dries extremely hard, waterproof and has a large elasticity coefficient and so is ideal for flexing. The only problem is that it is dear and it will cure once opened (because of the moisture in the air) and so use in small containers. Buy pints or quarts, not gallons. Quite often I will use only a half of a quart can, and in six months, if you then break though the skin with a chisel and hammer (about 1/4 thick by now), you still have half a pint or so of still good stuff. Spray is ok, but if you brush, it will dry brushmarkless. Use toluol to clean up, nothing else works as well. Once fully cured, 2-3 days, nothing will stick, primer, paint or even more POR, unless sanded with 600 and plenty of elbow grease. You can buy though POR primers and fillers that will etch it. If I am going to paint over it, I flash a quick spray of selfetch over it after a few hours dry time, and then it will stick and so will any top coats. I prefer it to an epoxy primer/filler. Good luck Bob.