If you are selling the truck, the new owner will appreciate it more if you don't mess with it. With the old paint, the new owner will be able to tell what condition the metal is in underneath. With primer, it could be hiding all kinds of imperfections. Given the choice between buying an old truck with faded paint and one thats got fresh primer, I'll take the one that's faded.

Paint & Body Shop moderator
A lone amateur built the Ark. A large group of professionals built the Titanic.