You are getting ahead of yourself. FIRST, find the extent of the rust. THEN, determine your course of action.

Regarding the primer (un-thinned), you can get away with quite a bit since the lower portions of the body are generally not finished to the quality of the upper fenders, bonnet and upper sides.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now