If it were a wooden door, moisture could also have caused the bubbles. In this case, however, it is a plastic door getting warm, and heat combined with the paint used could be the cause. This paint creates a more sealed surface, which means the heat trapped inside the door cannot escape. It would therefore be better to use water-based paint, i.e. acrylic paint. I suspect this is the same as with the iron gate, where all the paint is flaking off and only the undercoat, or at least, I assume the light green is the undercoat, remains. I didn’t paint it myself, by the way; it was an ordered and paid for job. It’s always very annoying when you have to try and correct other people’s sloppy work. In any case, the door has now been sanded down again; later I’ll see if a primer offers a solution, and then another coat of green. And of course, it needs to be painted when the door feels cold to the touch.