Primer/surfacer on a frame will be "interesting"- - - - -trying to sand on a frame rail is going to be quite an adventure, at best. I'd suggest using some kind of undercoater that will build up a thick enough layer to sort of hide the roughness. How many people are going to do a detailed inspection of the frame once the truck is assembled, anyway?
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!