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While you can remove the doors to paint them, leaving them in the door frame makes the job easier. Protect your floors by spreading a plastic tarp out under the doors. You can purchase a tarp at a home improvement store or wherever else paint supplies are sold. Cardboard and other absorbent scrap material can be used as a makeshift tarp. If your doors have any marks or dents, now is the time to take care of them. Rub the spot to wear away the material around it. Keep doing this until the damage disappears, then sand the surrounding areas to blend them in and smooth out the finish. If you still have trouble with these spots, try using rougher sandpaper or an orbital sander. Lightly rub the sandpaper against the doors to even them out and ready them for new paint. Go over every surface, including ones you treated with heavier sandpaper. The doors should look smooth and even once you are done. When sanding, always start with the coarser sandpaper. Then use the finer sandpaper to smooth the surface. Fill a bucket with about 1USgal (3.8L) of water. Mix in about a tablespoon of an all-purpose dish soap. Your regular dish soap works fine unless it is designed for harsher stains like grease. The water temperature doesn't matter too much. To stay safe, keep it cool or lukewarm. Dip a clean microfiber cloth into the soapy water. Get the cloth damp but not dripping. Squeeze out any excess water first, then thoroughly wipe the doors to remove all of the grime, oil, and sawdust that has accumulated over time. Get another microfiber cloth and go back over the doors. They have to be completely dry. This second pass should finish removing all of the grime that can ruin the paint job. Move onto priming right away while the doors are clean.
Lay a tarp under the doors. Buff damaged spots with 120 to 150 medium-grit sandpaper. Smooth the doors with 180 to 220 fine-grit sandpaper. Create a mild cleaning solution with dish soap and water. Dampen a cloth in the solution and wipe down the surface. Dry off the doors with a clean microfiber cloth.