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Phototec have you done this? I was just wondering if regular magnetic material is the opposite pole so they attract? Just a question/thought!
Actually no! My friend at another sign shop told me he has used this method on aluminum sided trailers, however I'm not 100% sure what base material he used. I do know that some Rubber Steel material only has interior adhesive on the back side, you need to make sure the backing material has exterior grade adhesive and is made for magnetic sign material to stick to the surface.
Some material has the white vinyl on the surface (for printing), I don't think that would be as good as the 30mil thick, that is just the flexible steel material with high-tac exterior adhesive on the back. From what I have read, the thick backing materials will hold the magnetic sign material as good as a steel truck door.
Signcrafters is correct, two pieces of regular magnetic sign material will not stick together very well.