Truly, there's not much you can do that will be durable. Any kind of spray paint just doesn't have the durability and will get worn down and torn off pretty quick. Regular automotive type paint would be more durable, but the real stuff needs to be baked on, which isn't really an option with the non-removable ball joints. Powder coating has the ultimate durability, but that too needs to be baked on, which isn't going to be good for those ball joints. Your best option is probably just use some cheap rust-inhibiting spray paint, and then go over it from time to time when it starts looking rough.