Yes, you can paint it. Would never have happened to begin with if you would have cleaned it like this each and every time since it was new: take a wad of paper towels and wet it really wet, put it in the microwave for two minutes. Take a dry wad of paper towels and put over the hot ones to remove them. Wipe out the hot damp microwave with the dry wad. Works every time and doesn't rust your microwave either.