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how to remove stubborn mold from deck after powerwashing

After power washing my deck I still have stubborn mold .  It is lighter but is visible and I dont if i stain over it will it bIeed through. 
I apply stain every 3-4 yrs and this is the first time that I mold that would not come
So is it ok to stain over it or do I need to remove it first and if so , how do I do this since the power washer wouldnt remove it ?
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Posted 2015-08-16T01:21:26+0000  by Nesi Nesi
Hi Nesi:

You are correect the mold should be completely removed prior to staining.

I have included below the solution to your mold problem from the North American Deck and Railing Association. This should solve your problem.

Nadra LogoNorth American Deck and Railing Association 
P.O. Box 829 
Quakertown, PA 18951 
Phone: 215.679.4884 - Fax: 888.623.7248

Why it Grows, How to Clean it, and How to Prevent it 
By Rick Trottier, Product Specialist and Key Account Manager
Correct Building Products, LLC

What is Mold?
Mold is the common term used to describe a growth on the surface of organic matter, caused by fungi, especially in the presence of dampness. Mold is mother nature's way of recycling organic matter. Every cubic foot of air has thousands of mold spores floating within it. Every surface - natural or man made, inside or out - has thousands more spores that have settled upon it. A fungus (pl. fungi) may be any of a large number of simple microorganisms that do not need energy from light for growth. The fungi bear microscopic spores that are produced in enormous quantities, are always present in the air, and spread via air currents. They are often water repellent and are resistant to desiccation (drying out).

"Mold" is often used interchangeably with the word mildew. These are the generic terms that describe a variety of microorganisms, including fungi, algae, rusts, yeasts, and bacteria.

Under a microscope, mold has an octopus-like structure. Its legs attach to a surface in a web-like manner. Once a mold has successfully colonized, it turns a dark color and releases thousands of new spores.

What Makes it Grow?
Sources of MoldMolds need three things to grow: food, water, and temperate weather. When the spores are in favorable environment, they will germinate. What constitutes a favorable environment is different for each species. Since the spores settle out of the air, they will be more prevalent on horizontal surfaces and are generally not found below overhangs, under porches or patios, or on the undersides of decks, even in damp areas. The colonies do not migrate, like a rot, but rather establish colonies and release more spores. The reason we recommend regularly sweeping the deck is the colonies are typically invisible; they only change color after they have released spores and died. Sweeping the deck removes the food source (pollen, dirt) and removes any dormant spores as well. After landing on a host material, a spore must obtain sufficient moisture to find enough food. Without moisture, the spores will lie dormant until favorable conditions occur.

There are many common food sources, such as dust, pollen, and other pollutants (see photo above). Water is essential for the growth of mold. Greater than 60% humidity, steady dampness, and temperatures between 40 and 100 degrees F, are the factors most conducive to growth. It will stay dormant if the conditions are too dry or too cold, or if there are insufficient food sources.

How to clean it...
It is important that any cleaning procedure fully remove the web-like "legs" of the colonies, penetrate the water-repellant outer surface, and remove the food source. The ideal cleaner is a steam-power sprayer, although normal power washers, or combinations of bleach, water, and laundry detergent are also effective. In all cases, use the correct protective equipment such as safety glasses, dust-mask and gloves. Since mold will die at temperatures above 140 F, we have had great success with a steam-power sprayer at 250 + degrees removing the mold. (See Photos B and C) 

Cleaning Mold Cleaning Mold

If staining is still present, follow with a sodium percarbonate product (OxiDeck) powder sprinkled directly on the stains, left damp for 6+ hours, and rinsed to help remove the stains (see photos D, E). Bleach, laundry soap and water can also be scrubbed onto the mold to help with the removal of the mold.(see photo F).

Cleaning Mold Cleaning Mold

Cleaning Mold

How to prevent it...
Preventing the growth of mold is as simple preventing the three factors that it needs to live. Keep the deck as dry as possible, and remove the food source by sweeping or washing. Here is a checklist of mold-preventive steps:

  • Gap the decking properly (minimum 1/8') for drainage
  • Assure that the gutters do not discharge onto the deck
  • Check that gutters are not backed up and overflowing onto deck dryer vent does not discharge directly onto or around deck
  • Pitch the deck properly away from the house for drainage (minimum 1/8" in 8')
  • Avoid the use of bark mulch around, on or under the deck (bark mulch is typically heavily laden with mildew spores)
  • Avoid the use of lawn or plant fertilizer on the deck (such as overspray from lawn fertilizer or Miracle-Gro from potted plants)
  • Sweep the deck every week, especially when pollen is heavy (typically spring and late summer)

Correct Building Products also recommends No More Mildew from National Allergy Supply as a spray-on preventive measure.

Posted 2015-08-17T17:42:31+0000  by Mike_HD_OC
Absolutely do not stain over mold, Nesi!

Mold or mildew creates blooms as it grows and these nuisance growths with unseat your stain and discolor the surface.

You need to remove it completely before applying stain.

Behr All-In-One Wood Cleaner contains diluted oxalic acid.

Not only will it kill the mold and mildew, but it will remove the discoloration from the surface of your deck.

Apply a generous amount of this cleaner to the surface using either a pump sprayer or a scrub brush.

Allow the product to work for about ten-minutes and then thoroughly rinse with fresh water from a hose.

Apply another coat to stubborn areas and scrub with a stiff bristle brush.

Rinse thoroughly with a hose after ten-minutes.


The only mistake you can make with this product is allowing it to dry on your deck.

It is common for stubborn stains to require more than one treatment.

If the stains are deeply embedded, don't overlook sanding the surface to remove the discoloration before applying stain.
Posted 2015-09-15T16:36:32+0000  by Pat_HD_ATL
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