try rhino liner or the like.
or if you have access to a 'farm' store, get some tractor or implement paint, use the hardener and it's tough and almost scratch proof, it's what most of us up here use for frame, underbody, rocker panels and bumper paint. different colors, both gloss and flat, are available Normally cost is under $20 per gallon, and hardener runs about $8. brush or spray, recomend naptha to thin.
ken.