Q: What can we do about staining on our painted wooden front door? It starts at the bottom of the door and moves upward during the summer. I’ve assumed it is mold, so I’ve tried cleaning it off with bleach, vinegar, a solution of Simple Green, and just plain soap and hot water. Nothing makes a bit of difference. The door (and the stains) have been painted over at least once. The stain returned.

A: You’re probably correct that the culprit is mold. Or it could be remnants of a water-soluble stain that got onto the door at some point. Whatever it is, it looks like it is underneath the paint, so scrubbing the top surface doesn’t touch it.

Paint is a great hide-all covering. Besides adding pizazz to surfaces like your front door, it blocks ultraviolet rays from the sun, thus preventing damage to the wood underneath, and it creates a slick surface that sheds rain, which also helps protect the wood.

But what’s not so obvious is that paint does not completely seal the surface. Water vapor passes through water-based finishes fairly easily, which is ordinarily a good thing because it allows the wood to dry, rather than rot, if it becomes damp. But if mold was growing on the wood when it was painted, the mold can regrow. It may happen most during the summer because that is when condensation can form where the door, cooled by air conditioning indoors, bumps up against hot, humid outdoor air. As that moisture evaporates through the paint layers, mold stains or other water-soluble stains also work their way up through the paint layers, creating the blotches you see.

The quick fix would be to repaint the door with a stain-blocking primer and then repaint. That should keep stains from showing through the newest paint. However, it would not address an underlying mold issue, which could eventually cause the paint layers to peel. Only stripping and starting over would prevent that. To test whether the paint is already compromised by mold growing underneath, run your finger over the stained areas. Is the surface there just as smooth as surrounding areas? If so, you might want to opt for priming and painting one more time. If the spots still reappear, you’ll know that you need to strip.

If you decide to tackle the chore of stripping and repainting yourself, follow instructions on whatever stripper you use. If the door was installed before 1978, the paint might contain lead; follow lead-safe advice from the Environmental Protection Agency. It’s easiest to work on a door that is off the hinge and flat on sawhorses or a worktable, but if you opt to strip with the door in place and don’t need to worry about lead dust, cover the floor completely so you don’t wind up dripping stripper or gooey paint on the flooring.


While stripping, you may see evidence of mold growing between layers of paint but not between the wood and the base layer. In that case, the wood will be ready for priming and painting once all of the residue is cleaned away. If the wood itself is stained with mildew, wash the wood with a solution of one part liquid chlorine bleach to three parts water, or apply Zinsser’s mold killing primer. Unlike typical primers, it is registered as a protective coating by the EPA and is allowed to state on its label that it can be used to paint over and kill existing mold. The label also says it prevents new mold growth on the paint film, a claim that manufacturers of numerous other paints also make.

If you use bleach to kill any existing mold, brush on primer once the door dries, or select a finish paint that is self-priming. Water-based exterior primers and self-priming paints generally work well. But whether you use the mold killing primer, a standard primer or a self-priming paint, inspect the surface after the initial coat dries. If you see any stains, cover that first coat with a stain-blocking oil primer, which should block all water-soluble stains. (If the door is redwood or cedar, which are high in stain-causing ingredients known as tannins, save yourself a step by just using oil-based primer from the beginning.)

Whatever preparation steps you do, it’s important to get finish coats of paint on the door relatively quickly. Plenty of mold spores are sure to be in the air, and if they land on the door and there is enough moisture for mold to grow, you will be back to again having a surface that isn’t ready for paint. The Zinsser label says that mold killing primer can be re-coated in one hour and that it should be done within 72 hours.

For the finish paint, choose a product that stands up to scrubbing — doors are magnets for fingerprints — and is labeled as resistant to mold growth. If you want to use the same paint on the inside surface of the door, buy paint labeled for interior and exterior use because some exterior-only paints contain mold inhibitors that aren’t safe for indoor use, said a customer-service representative for Sherwin-Williams. She suggested using Emerald exterior acrylic latex paint, which has a self-priming finish, if you are painting only the exterior. If you want to use the same paint on both sides of the door, she recommended Emerald urethane trim enamel, which needs a primer.

She also suggested one step that might provide additional information: If you bought the earlier paint from a company store, you might be able to get a representative to visit your house and offer advice. Sherwin-Williams’s sales reps make house calls to help diagnose paint problems and recommend solutions, she said.