New rust under primer

[73resto]

I was sanding the primer on my rear hood that i primed a while back and uncovered some new rust. What the heck. a small area was sanded down to bare metal before i applied the primer . why is this rusting
does this mean that everywhere that i sanded to bare metal is going to rust again under the primer with out me knowing?????

[Campy]

What type of primer did you use? Bare metal has to have epoxy primer (waterproof) on it. Other primers, including primer sealer, are not waterproof.

[redbug64]

Primer absorbs moisture. If the car was outside, or in a damp garage, for more than a day, you prolly sucked in some water. Id check it over, etch prime where needed, get a good prime coat, sand it and get some paint on there. Some "etch" primers will absorb slower, but you definatly dont want to be driving the car aroudn with just primer on it.