I am shooting for a gray military scheme, so I simply painted the interior with gray Deltron primer and then topped it with 2 coats of Deltron 'flexed and flat' clear. Turned out pretty good - it's cheap, fairly light weight, and easy to spray. I know it will be durable since I did a beta test of the same treatment on the tailgate of my old pickup last spring. It has held up very well under hot sun, rain, hail, blizzards, etc.
