Black Spots on Exterior Walls – What Are They and How to Remove Them

Have you found black spots on the exterior walls of your home or cottage? The spots are an unpleasant surprise, but don’t worry: black spots on exterior walls are most likely harmless surface mold. While mold sounds alarming, there’s no need to panic in this case, as removing mold from wood is not difficult. In this article, we’ll explain how to easily remove black spots from exterior walls using simple home methods.

What Are Black Spots on Exterior Walls?

Black spots on exterior walls are usually surface mold, which is harmless to both the building and people. The spots are primarily just an aesthetic problem, so while the idea of mold might be alarming, in this case it’s a harmless matter.

The spots are primarily just an aesthetic problem, so while the idea of mold might be alarming, in this case it’s a harmless matter.

Marks caused by blue stain fungus sometimes resemble black mold spots. Blue stain can be identified by the fact that while black mold spots are only on the wood surface, blue stain fungus penetrates deeper into the wood. You can check this by scraping or sanding the discolored surface: If the spots are only on the surface, it’s most likely surface mold. If the wood is discolored deeper down, it’s probably blue stain.

Blue stain fungus is also considered primarily just a cosmetic problem and has no effect on wood structures.

What Causes Black Spots on Exterior Walls?

Over time, mold spores floating in the air attach to wall surfaces. The wind carries pollen and other organic matter from nearby trees and bushes, which helps the mold spores grow. Gradually, spots begin to appear on the wall surface.

Black spots thrive especially on north-facing walls: a favorable surface for the spots is a shaded wall where moisture remains longer than on a sun-exposed surface.

Black spots thrive especially on north-facing walls

Spots can be found on both painted and unpainted wood. Walls treated with oil-based paints generally collect spots more readily than walls painted with water-based paint.

How to Remove Mold from Wood?

There are essentially three options for removing mold spots: washing with water, detergent, or mold remover.

Usually removing mold spots is easy: often the spots come off easily with just water and a brush. If the spots seem stubborn, you can use a detergent, such as Tikkurila House Maintenance Wash. However, water and detergent washing are not sufficient to remove all mold microbes, so they remain on the wall surface invisible to the eye. The spots will therefore most likely return sooner or later, but water or detergent washing is an excellent way to clean the walls between maintenance painting sessions.

If the house is being painted, you can and should use a mold remover, as dirt and mold spots must be removed before applying a new paint surface. For example, Tikkurila Mold Remover is a suitable product for this.

Mold remover is strong, so when using it, plants near the wall should be protected – especially edible ones. Since the purpose of mold remover is to kill organic mold microbes, it will also harm other plants unless they are properly protected.

Mold remover is strong, so when using it, plants near the wall should be protected

Detergents and mold removers are diluted according to instructions and applied to the wetted wall from bottom to top, after which they are allowed to work for the time specified in the instructions. The products can be applied using a brush or garden sprayer, for example. The products must not be allowed to dry, but the wall must be kept moist during the working time. After this, the surfaces are rinsed using a brush.

You can read more about washing exterior walls in our article Washing Exterior Walls Before Maintenance Painting – How It’s Done.

Can You Paint Over Black Spots?

Black spots must be washed off before applying a new coat of paint. It’s recommended to use a mold remover. If you paint over the spots without cleaning them, they will appear very soon. Additionally, unwashed dirt weakens the adhesion of the new paint surface, causing the new paint to peel prematurely. Removing mold from wood and removing dirt are essential parts of a house maintenance painting project.

If black spots are looking for someone to clean them and your home’s paint surface needs an update, we’ll gladly do the work for you. By requesting a free assessment visit from us, you’ll get our expert’s evaluation of the necessary measures and a quote for the work. The assessment visit is always free and doesn’t commit you to anything. We do the work with experience from over 4,000 projects and naturally provide a guarantee for our work.