They make two kinds of spray undercoating for the frame that I have used. One is a rubbery type of stuff that will harden, but takes a lot of time. for the first few weeks it is very sot and pliable, but then becomes like a very hard rubber.
The second has texture like the rubber stuff, but hardens like an acrylic, or epoxy. I have used the rubber stuff first, then the hardened texture stuff next and it works pretty good. It is flexible so it doesn't chip or crack, but the second layer is hard so it doesn't gouge or scrape off.
Finally a month later I put a high gloss black on it to help keep the mud from sticking. Now it is well protected, does not chip, and cleans very easy. Mud slides right off. Re-apply the high gloss as needed.
armsup
Oh, be wary of the stuff just labeled plain "undercoating spray". That crap sprayed on like a damned tar. It was like the stuff royce speaks of, but it would stick to my skin and clothing if I scraped up against it while working under the rig. That stuff sucked !!
Now I always do a test patch on apiece of scrap metal and let it cure for an hour or tw,o to test the finish before I spray the while thing or big parts.
Shoot... I would even use bed liner in a can, as long as it was sprayed over with high gloss after it was done. The high gloss keeps stuff/dirt/mud from sticking like it dose to most bed liners.