Eastwood has some paint you can use. I think it is paint used for car frames and it is great. I think it maybe an epoxy paint, I know my Dad used some on a Frame for a 49 DeSoto. I took a Steel wire brush to it and it would not scratch up. It need to set up for a few days but after that it was hard as metal. Hope this helps..
Car quest sells something called tractor black in a spray bomb, it dries hard and is corosion resistant, its the best spray bomb you can get. If you have a spray gun and compressor spies hecker makes a rally matt black that is catalyzed and is second best only to powder coating. Spies hecker is expensive but good and user friendly