If the door is especially dirty, add ½ cup of vinegar to break up the grime. Then scrub with a fresh sponge, microfiber cloth, toothbrush, or paintbrush and dry it with a towel.
To clean fiberglass doors, combine one cup of plain white vinegar and a gallon of water into a spray bottle and use a sponge or lint-free cloth to wipe.
It's important to check steel and metal doors for rust regularly, and if there is, soften it with an old towel dampened with kerosene and scrape it away with sandpaper.
Stainless steel doors must be cleaned with glass cleaners without vinegar or ammonia or simple dish soap mixed into hot water, as other cleaners can leave scratches.