Many exterior cellar doors are horizontal, like trap doors into your cellar, crawl space or basement. Since horizontal surfaces are especially vulnerable to full sun and sometimes standing water or snow, a paint suitable for other horizontal surfaces, such as decks, will hold up better. Metal cellar doors need to be treated for rust and coated with rust-inhibiting paint, and wooden cellar doors should be painted regularly so they don't rot.
Scrub the cellar door clean, using mild detergent. If there is mildew or moss, use a mixture of 1 cup of bleach to 3 cups of water or a commercial deck or siding cleaner containing mildewcide. Rinse the doors well. If the doors are made from wood, do this at least two sunny days before starting to paint to allow the wood to dry.
Scrape off loose paint and rust, and sand the doors with 50- or 80-grit sandpaper to feather the edges and make it smooth. Use a power sander to get the job done faster. A wire brush designed for cleaning barbecue grills also works well for removing rust.
Pretreat any remaining rust with rust stabilizer. Rust stabilizer is a water-based solution that soaks into rust and halts the rusting process. You can find it at most hardware or automotive supply stores. Follow the directions on the label.
Use a bonding sealer that inhibits paint from peeling on old, wooden cellar doors with multiple layers of paint. These sealers "glue down" old paint and seal small cracks and fissures to prevent water from getting in.
Prime the door with an appropriate primer. Use either water- or oil-based exterior primer for wood or oil-based primer for metal. Use two coats of rust-inhibiting oil-based primer if you have not applied rust stabilizer.
Caulk or fill holes, gaps or remaining cracks with paintable silicone caulking for wood. Use an epoxy filler for dents in a metal door. Don't use Spackle on horizontal surfaces – it is too porous and will start cracking and crumbling within a year or two.
Paint the cellar door with two coats of quality 100 percent acrylic deck paint. A satin or semi-gloss paint will hold up better.