Can You Paint Over Green Algae?

Can you paint on top of algae?

There are few things as disheartening as seeing green algae and mold growing in the grooves and crevices of your timber decking, fences, siding, or outdoor sheds.

It can be tempting to simply rub a bit and paint over the algae so that it is no longer seen. But is it a good idea? No, actually it is not!

While it may seem at first that painting over the algae will deny it the oxygen needed to grow. The truth is that the spores will continue to live and grow over time beneath the paint.

This means that simply painting over them is only a temporary solution. If you want a long-term solution, you must find a way to remove the algae altogether.

But keep in mind that scraping off the old paint and algae will not always be the complete solution.

The green algae may penetrate the surface to the point where it hides deep enough not to be removed by scraping alone. So, you will need to use an effective cleaner or treatment designed to destroy the algae, moss, mildew, and mold.

And once you use the cleaner, you may find that you do not have to repaint the surface at all if the inner layer is still in good condition.

Will Paint Cover Algae Growth?

Many brands of outdoor paint indeed have formulations that either slow the growth of algae or prevent it from growing at all.

However, it does not destroy the algae or mold. This means that eventually, it will come through the new paint.

The only advantage of simply painting over the algae on walls or wood is that it will, for a time, cover the green stains.

But within a matter of weeks or perhaps a few months, the familiar green stain of algae will return. So, all that effort you put into a solution does not last.

How Do You Get Green Algae Off Painted Walls?

Here are a few simple steps that can help you remove algae buildup on your walls…

Step 1- Spray Cleaning Solution:

While you can use plain water and some elbow grease to remove much of the algae from the surface, it will not be enough to get at the algae which have settled in the cracks and openings.

To treat such areas, you will need to use stronger products that destroy the algae on contact. For light algae, consider the following mixtures:

  • vinegar and water
  • liquid chlorine or household bleach with water

These solutions can be mixed and used in spray bottles without any professional help. You just need to be careful to use the right mix of components.

For vinegar and water, the right ratio is one part vinegar to two parts water. And for chlorine or bleach, the ratio should be one part of chlorine to four parts of water.

Step 2- Let it Sit and Scrub:

After applying the solution, allow it to sit for a few minutes before gently scrubbing it off using a brush or sponge.

Begin from the top and gradually work your way down. If needed, employ a scraper to effectively eliminate any persistent algae and mold buildup.

Step 3- Rinse and Spot Clean:

After removing all algae, mold, or dirt, proceed to rinse the exterior walls with a water hose or pressure washer. Thoroughly inspect the area for any lingering signs of mold or algae.

In case there are still remnants of green algae, utilize a brush and cleaning solution to scrub them away. Finally, rinse off any remaining traces of the cleaner with plain water, ensuring a spotless finish.

painting over green algae on fence

What to Use to Remove Algae from Wood Surfaces?

Removing green algae from old wood surfaces (whether indoor or outdoor) is important if you plan to repaint them for a fresh look.

1- Oxygen Bleach:

You may choose to use a specialized deck cleaning product, but bleach is cheap, quite effective, and can be found in the laundry section of most retail stores.

In fact, many specialized deck cleaning products use oxygen bleach, or sodium percarbonate, as it is sometimes known, as its main ingredient.

  • Mix 7.5L of warm water with two cups of oxygen bleach.
  • If your deck has grime, add ¼ cup of liquid dishwashing detergent.
  • Go ahead and clean the deck with a brush.
  • After 15 minutes, rinse off the solution with water from a hose.
  • Repeat this process as necessary for stains that are left.


Do not confuse oxygen bleach with chlorine bleach. Chlorine bleach may cause damage to the timbers of your deck. While some recommend chlorine bleach, oxygen bleach is far less damaging.

Keep in mind that some bleach solutions may alter the color of the natural timber. So, before you apply the solution to the entire deck or fence, try it on a section that is not seen to test out its effectiveness.

2- White Vinegar:

White vinegar is quite an effective algae killer, but it also requires considerably more scrubbing compared to oxygen bleach.

  • Prepare a mixture of white vinegar and water in a sprayer bottle
  • Spray it over the deck thoroughly and leave it there for 10-15 minutes
  • Then rinse away using clean water from a water hose or pressure washer.

If required use a brush to scrub away the algae stains that are left behind.

Note: Another great natural option to use for algae removal is baking soda. You can make a paste out of it and apply it directly onto the affected area, scrub it off with a brush, and rinse away the residue afterward.

3- Fungicide:

Many popular brands of commercial fungicide sprays are now available to kill algae or mold without damaging the grass or plants nearby.

Plus, when you do not rinse away this environmentally safe solution of fungicide, it will continue to kill any new algae or mold that might settle on the deck.

  • When spraying on the fungicide, you must leave it on for the designated time before repainting the deck or surface.
  • For green algae, it will generally mean a few days. However, if you have black mold, you may need to leave it on for around an entire week or even a month.
  • Once the algae have disappeared, lightly rinse the area before you can start the painting process.

Note: If you are considering using a chemical cleaner, read the label carefully as some may be damaging to wood surfaces if you do not use them according to the instructions.

4- High Temperatures:

Removing green agae from outdoor furniture and other wood surfaces through steam cleaning is another option. The power of steam can effectively break down the molecules in algae, killing it in the process.

The temperature you should use for steam cleaning could range from 150°F to 250°F (65°C to 121°C), depending on the surface material and stubbornness of stains. However, higher temperatures come with risks as they may damage certain surfaces, such as attached plastics, hence it is best to consult the manufacturer’s instructions first.

What Types of Paint Can Be Used to Cover and Treat Algae?

For slower-drying and humid areas, it’s a wise choice to invest in paint that actively resists algae.

These algae-resistant paints are particularly suitable for decks or walls in shaded areas with limited sunlight. By using them, you can effectively prevent algae from taking hold and maintain the aesthetic appeal of your outdoor spaces.

Cabot DeckCorrect is one such product that offers excellent algae resistance. It comes in multiple colors and can be used to cover old wood or concrete surfaces with ease.

Another great option for preventing the growth of algae is Rust-Oleum’s Deck & Concrete Restore 10x, which contains a powerful fungicide and algaecide.

Some of the algae-resistant paints do grow more porous as time passes. This means a new coat will help prevent moisture from forming on the surface which will keep any new algae or mold from gathering on the paint and creating more green slime.

In addition to paint, you can find many sealants, oils, and primers that also have properties that resist algae growth to keep the surface in like-new condition for a long time. Such products can also help bring out the best in the natural color of the wood used for your decking.

Related FAQs

Will algae rot wood?

No, algae do not feed on timber. But if algae are present, it means enough moisture exists to cause the wood to start rotting.

Fungi and bacteria also thrive when moisture is present on wood, causing such microorganisms to reproduce. The result is that the wood begins to fall apart.

This is why you should treat the algae as soon as it is seen. Plus, you should take steps to identify and correct surface moisture issues to prevent rot.

Can algae be dangerous?

In an indirect sense, algae are certainly dangerous to wood and wooden garden furniture.

This is because the growth of algae is fueled by moisture building on the surface.

It is the moisture itself that represents the danger to the wood as it causes rot and structural damage.

If you see algae, it’s a warning sign that the wood is in danger.

In addition, the growth of algae and mold can make the deck surface slippery to walk on.

And while most algae forms are only a minor nuisance to humans, the spores from more dangerous forms can be problematic in terms of your health.

Where can I find algae in my house?

Algae is usaully formed due to moisture in the environment, meaning that it can be found around areas of your home that are often exposed to water and dampness.

These include:

  • Basement walls and flooring
  • Kitchen and bathroom tiles
  • Landscaping stones and bricks
  • Swimming pools, hot tubs, and pool decks
  • Wooden fences or even metal surfaces such as pipes and gutters
  • Any surface that is kept in direct contact with soil or grass (e.g. garden furniture kept outdoors or in humid environments)

You will most often see algae forming on the north side of the home in the northern hemisphere. This is because the sun does not directly shine on the north side, meaning its ultraviolet rays, which destroy algae and mold, are not directly applied.

The Bottom Line

The darker and more humid the conditions, the more likely algae, and mold will form on your porch and home’s walls, siding, or timber.

To avoid algae growth, you need to take steps to reduce moisture on surfaces or apply paint with anti-algal properties. Plus, where possible control the amount of sunlight and shade that hits the area as this can help prevent future growths.

It’s also wise to regularly check for any signs of new risks such as green slime on your deck or other surfaces, and take steps to treat it as soon as possible before painting over it.

By taking these measures, you can keep your outdoor living spaces looking their best while avoiding the risk of damage caused by algae growth.

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