Stain coming through paint
I am renovating my apartment, which was built in the early '60s I believe. This place has metal doorframes on all the interior doors, similar to an office. It's already a pain in the neck, because the hinges can't be replaced (one half is molded into the frame somehow) and the striker jam (I think that's the right name) is pre-cut into the frame, which has settled over time so a lot of the doors didn't shut properly. But that's not my problem.
On three different doorways, I used a heat gun to get rid of the layers of paint caked on the frame. When cool, I then sanded the frame, primed, and painted with two coats.
The next day, I noticed tiny brown spots on one frame. The best way I can describe it, it looks like something was splattered on the frame, like Coke or blood or something. They range in size from pinhole, to about half the size of a pencil eraser. I found this strange, because I knew it was not possible. The surface was also still smooth. So I put a little paint over the area again, and still, the stains are coming through.
I used the same brushes, primer, and paint on two other doorframes in the house at the same time, and the other two don't have a problem. This one, however, is right next to front window, so is exposed to sunlight. Could this have something to do with it? I just find it hard to believe that even rust would come through that fast.