Mold, Mildew Painting house trim

Published on January 15th, 2017 | by Henri


Mildew builds up on house trim

Q.  We had the stucco and the trim repainted on our house five years ago. In the last two years, we have noticed mildew on all the white trim and have had to hire a crew in the spring to clean our two-story house. They use a bleach and water mix and it immediately cleaned the trim with a little light scrubbing. No mildew appears on the stucco.

Having never had this problem before at our prior home or the first five years of owning this home, we are wondering why this is happening. We had all the trim cleaned early this summer and it is already back, making the paint look sooty and terrible. Is there something different about the paint or is it environmental?

We have not added any mulch, which I know can cause issues you have addressed in the past. We did not overwater and actually have fewer trees around the house than when the house was purchased. We notice our neighbor, who lives in a matching house and has not repainted as recently as we have, is having the same problem—the trim is sooty in appearance and the stucco is not.

A. Your problem may be environmental; among the possibilities are that the paint used may have a linseed oil base instead of a synthetic oil base, and did not contain a mildewcide; or the surfaces were not thoroughly cleaned in order to kill all polluting spores. Since the stucco was also painted and does not show the same signs, it is unlikely that paint with linseed oil was used. And it is possible that the stucco dries fast enough to discourage the formation of mildew, whereas the trim does not.

So I am leaning toward the possibility that the trim may have been repainted over a compromised surface already polluted with airborne particles. A bleach solution would not by itself remove other pollutants; it would require the addition of TSP-PF to the bleach solution.

Do you live near windy agricultural fields, an industrial site emitting smoke, or a body of water? The stains on your trim are not a case of artillery fungus from decaying organic mulch, which manifests itself by small, dark dots on siding that are hard to remove.

A better cleaner than a bleach solution, for this purpose, is a mixture of equal parts Oxy-Boost and EXTERIOR PROx Nontoxic Deck & Patio from Ecogeeks. You can buy both at

Since you mention mildew and the fact that it gets cleaned with bleach, you may want to consider painting the trim with Zinsser’s PERMA-WHITE Mold & Mildew-Proof Exterior Paint. Visit, and click on Find a Product, followed by clicking on the Zinsser’s icon. You may be able to find these products in paint stores and well-stocked hardware stores.

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