Seven Common Questions About Exterior Painting – Black Mildew

Example of Black Mildew on Exterior Surface

Black Mildew

Seven common questions about exterior painting

Why is my light color trim or siding turning black? Mildew?

This question is a follow up to the previous one we had talked about.

Many people notice black dots or sometimes the entire surface turning black within a year or two after their house was painted. Mildew is common in the north east with plenty of rain and humidity that provide an optimum condition for mildew growth.

However, the problem is exacerbated with not cleaning the mildew with proper mildew cleaner before painting the house. More importantly, use of the wrong primers can cause mildew growth. As a matter of fact, certain organic ingredients in the oil primers and paints act as food for mildew.  

Mildew built-up on the exterior of the house caused as a result of oil based stain used on the house.
Mildew built-up on the exterior of the house caused as a result of oil based stain used on the house.
Example of Black Mildew on Exterior Surface
Example of Black Mildew on Exterior Surface

How Oil Primer Leads to Mildew Growth

Many painters use oil primers as the primer of choice for all their exterior full priming or spot priming. In fact, many painters also make customers believe they are providing a better job of painting their house. Usually, it’s a painter dealing with peeling paint or sealing surfaces. Furthermore, they use oil primer in hope of fixing a problem but they unknowlingly introduce another.

Because of the specific organic compounds in oil, mildew begins growing and multiplying under the painted surface and makes the house and trim look dirty! In some cases, I’ve seen houses that turned black a couple years after they were done.

Evidently, with the advances in paint industry in the past twenty years, we have alternative paint products to use to deal with the mildew issue. There are better cleaning surface and mildew products. Use 100% acrylic primers and mildew additives for exterior painting. In addition, Mildew sealers/blockers available that can be used to prevent or deal with mildew on painted house and trim.

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