what primer

is there a primer tht will prevent rust

if you have rust you need to cut it out and weld fresh sheet metal in,
i dont wanna say anything bout primer cause ii hear 2 things!

for bare metal that has no rust..you should spray it with a zinc rish self etching primer..then follow with an epoxy or catalyst primer..a lacquer primer will allow moisture through and rust will form....these things will prevent rust but will not stop rust if it has already started....

epoxy will hold the moisture out. paint as in base coat does nothing to prevent it but the clear being a solid will.

Bare metal needs to be epoxy primed before anything else epoxy sticks to metal best,etching primer is old technology use epoxy

I could be wrong..but..I think primer does keep moisture out..and does completely seal metal..it just doesnt hold up to the elements as well as paint does..other words the primer will break down before paint will..but generally epoxy primer does hold out the moisture..