There is your problem. No matter what type of paint you use, it will not adhere properly if the surface isn't clean. You need to bust out the grinder with a roloc disc and grind it to bare metal, clean it and shoot it with hi-temp.
Look at it this way; at least they're being kept on the road and not crushed.
I like stock Model A's, but they're not my passion. Honestly, I love hot rods and customs. I feel like nothing looks cooler than a chopped model A, and it's not because I can't afford a 32, or to restore an A to...
I had the same issue too. The guy I bought it from globbed on about a half a tube of RVT and wrapped a piece of a robe around it...
I pulled it out and took it to the radiator shop and they charged me I think $30 to clean it off, find the leak, and repair it.