I've now seen two metal front doors that have large, growing blisters of paint, both of which had security magnets drilled into them for the home security system. Can the magnets cause a chemical reaction that causes the paint to blister? I tried to help my neighbor a few years ago to repair the blisters. The blistered areas were sanded to the metal, and painted with one of the major brands that was listed as rusty metal primer. To my surprise, the rust began to reappear as the paint was drying! The job failed, and my neighbor mentioned that the security system was installed a few months before the blisters appeared. Could there be a connection? I was recently asked to repaint a door for someone in another home that had the same occurrence of random blisters. This door also had an alarm system with a magnet drilled into the door.