By far the most durable "paint" you can get is POR-15. I believe it is polyurethane based. Its not at all cheap, but its really worth it for certain projects. You can get it on line from them.
here:
http://www.por15.com/
It is also sold like Amsoil is, by local 'distributors'.
The car restorers use this. I have it and have used it a number of times. I'll tell you, there is no other paint that even comes close to how tough this stuff is.
Ken