Ask the Expert Question-and-Answer Archive

by Ron Joseph

October, 2004

Determining Whether Substrate is Steel, Aluminum or Plastic (Fiberglass)

Q. How do I tell if my door is aluminum, steel, or fiberglass. I would like to repaint it, but I don't know material I am dealing with. It looks like it has been painted already, however, the frame around the outside window was left unpainted, and it looks like some sort of plastic. Is there someway I can tell? Can all of the above types of doors be repainted?

Can you find an area on the door, perhaps near the hinges out of view, where you can scribe a line using an "Xacto" knife or a box cutter? Carefully make a cut through the paint and all the way to the substrate, If the substrate is shiny and metallic then it is clearly not a plastic (fiberglass).

If the door is made of steel it will be magnetic. Therefore, if a kitchen magnet stays put on the door, you know that the substrate is steel. Depending on how many layers of paint have been applied you might need a stronger magnet than the ones that are used on refrigerators.

If the doors have already been painted, you can repaint them, no matter what substrate they comprise, steel, aluminum or fiberglass. If you need to paint over the bare substrate, your local hardware store can supply you with the most appropriate primer. If you are not painting over the substrate, you can use any of your choice, and the folks at your hardware store will give you the appropriate primer (if necessary) and topcoat.

Best wishes,

Ron Joseph