Updated February 2020
Black Mold on Deck Stains
We appreciate it when people write in and ask for advice when something goes wrong during their deck staining projects. Today we talk about what makes your deck prone to black mold growth after application and how you can prevent it. We appreciate your input, so feel free to leave a comment below with pictures of your deck stain projects.
Black Mold and why it Grows in Certain Deck Stains
Exterior deck stains are subjected to the elements making them prone to natural contaminants. A deck stain is there to shield the wood from harsh weather and the damage it can cause such as rot and decay. In providing protection sometimes the deck stain itself can be compromised.
Black mold is one of the natural occurrences that can affect wood stains effectiveness and longevity. There are several reasons why black mold can grow on or in a deck stain but here are the most common reasons.
Over Applying Deck Stain
When a deck stain is over-applied to the wood it can affect the curing or drying time of the stain. When this happens the deck stain remains tacky for several days or longer allowing dirt and tree pollen to become embedded. Once these contaminants have intruded the stain they can attract and begin to grow black mold even after the stain finally cures.
Tips: To avoid curing issues by over applying a wood stain, work in small areas or one board at a time. Let the stain soak into the wood for several minutes then use a brush or a stain rag to wipe any excess drips or puddles.
Inexpensive or Cheap Deck Stain
Going with a cheap deck stain may save you money initially but may cost you more in the long run. Cheaper deck stains consist of a cheap grade linseed oil that lacks the mildewcides that help fight black mold.
Tips: Do some research and buy a quality deck stain that is not prone to mold and mildew.
In some areas, black mold is almost impossible to avoid. Hot and humid climates or areas next to water are always more prone to all sorts of mold, mildew, and algae. In this situation, black mold can grow on top of a deck stain and jeopardize the overall appearance.
Tips: In this case, use a deck stain that fights against these contaminants while increasing the frequency of deck maintenance to keep black mold from becoming a problem.
Oil-based stains contain both natural and synthetic oils. These oils can feed the growth of mold “in” the stain itself. All stain manufacturers add mildewcides and algaecides to help prevent this. Some stains like TWP have an EPA registered wood preservative ingredient.
Tips: Use an oil-based stain that has a proven track record of not attracting mold and does not turn black in color.
Water-based stains do not feed the growth of mold like an oil but still can attract mold once the stain starts to fail.
Tips: Choosing a water-based decking stain with zinc oxides will minimize mold.
Have a question or comment about black mold and your deck? Please ask below.