Unfortunately, removing paint from metal surfaces isn't easy, but it can be done with a lot of elbow grease and the patience to match. Don't expect to simply wipe the paint away from your metal door; instead, expect to invest a lot of time and to put in many repeated attempts before the job is finished. Also, keep in mind that if the paint surface is smooth, it might simply be easier to paint over your metal door than to strip off the paint.
Prepare Your Workspace and Yourself
Because stripping paint from metal can require the use of caustic chemicals, it's best to remove your door and bring it outside or to an open garage or shed with good ventilation. Lay out some drop cloths, cardboard, or newspaper in your work area to protect the ground from the chemicals. Remove any flammable surfaces from the area.
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Put on chemical-resistant rubber gloves, safety goggles, a dust mask, a long-sleeve shirt, and pants to protect yourself.
Apply a Paint Stripper and Remove Paint
Fill a metal can or glass bowl with a chemical paint stripper and then apply a thick layer to the door's surface with a disposable paintbrush. Leave the stripper on as long as is directed on the manufacturer's instructions. Different products recommend different time frames, which could be anywhere from 30 minutes to overnight. Eventually, the paint will start to bubble.
Use a wide-blade steel paint scraper to remove the remaining liquid and paint from the surface as much as possible. For smaller, harder to reach areas, you may be able to use a scrubbing pad or toothbrush.
Not all of the paint will be removed with the first application of paint strippers. If the first application doesn't work, apply more paint stripper, wait for the recommended time period, and then scrape away more paint.
Try a Heat Gun
While paint strippers and scrapers are effective over time, you can also try a heat gun. Put on heat-resistant gloves and then hold the heat gun a few inches from the surface before turning on the gun to its lowest setting. Move the heat gun back and forth slowly, focusing on a small area at a time. Make sure to avoid touching the metal surface because it will get dangerously hot. Just as paint stripper will cause the paint to start to bubble, so will the heat gun. Once the paint starts to bubble, carefully use a paint scraper to remove it from the surface.
Use an Angle Grinder
While hardly the quietest method, using an angle grinder to strip paint off the surface of metal might just be the quickest and easiest method. Look for a strip disk that is designed to remove paint from metal so it will not damage the surface. It's worth noting that while effective, an angle grinder is best used for large, flat surfaces, so any detail work on your door will still require either a paint stripper or heat gun.
Apply Mineral Spirits and Water
When the paint is removed, wipe the surface with mineral spirits to clean off any paint flakes that fell back on the door during the paint removal process. This will also remove paint stripper that was left behind. Next, rinse off the door with a garden hose to ensure all paint stripper has been washed away. Finally, dry the door thoroughly.