Its actually easier to just sand it off. I have done a few of those and we just sand as much as we need to the peeling paint off. Prime the peeling areas, sand and paint the truck. You could sand it all off down to the factory primer, but thats alot more labor and it will start to get even...
Unfortunatly, best case would be to do a full repaint. Prep and prime the peeling areas properly and refinish the whole thing. You could just do spot jobs on the bad areas, but if it happens elsewhere you will have to continue to chase it..
The only reason i question it is because it looks like where it flaked off i see more white paint underneath. Its usually green ecoat primer you see. I paint cars is why i was asking...