Here's a truck bed coating worth a look in a paint on bed liner. See some of the down sides of bed liner paint are pretty much the same with all the brands. That goes for DupliColor as well as Herculiner and Plasti Kote liner paint.
For one the stuff is a major mess. Tools might as well be throw away because that's about all one can do. The liner paint is a serious mess to clean up.
Most likely the best approach is to use primer on the to-be-painted surface as that is the best way to make sure it, the bed paint, stays on and not peeled off and away...
But then there's What A Liner.
I haven't tried it yet but they surely have some good sounding features.
WhatALiner is water soluble. That makes for some easy clean-up, a mighty large plus in my eyes. But there's more.
The claim is that no primer is needed and it can even go on over rust. How good is that???
Now for sure all the loose stuff has to come off and that means some scraping maybe but nothing more other than getting off the grease, oil and wax with solvent cleaner. It looks like a good deal for a do it yourself spray on bed liner, or more correctly a roll on bed liner...
Check out the pictures of the truck used by a contractor for 5 years. Very impressive. May be worth a try... Get a truck bed coating and a roll on bed liner.