Mold & Mildew Stained Composites

Mold & Mildew or other fungus growth, such as common green lichen or moss (often confused with algae), and the stains that result are considered an “Act Of Nature” because fungi can only occur in environments where spores exist, they land on a surface and receive moisture, such as in the outdoors, where composites generally are located. This is generally true unless composites are made of recycled wood fibers and/or recycled plastic where molds were manufactured into the composites. Due to this fact, composite manufacturers generally clearly disclose  that “mold & mildew” and/or “fungus” can be an issue in their product literature.


Picture of a neglected PVC “CAPPED” composite deck growing a green & black mold “biofilm”.

The second generation of composites, which are generally made from PVC exterior “CAP”, with a recycled wood & plastic core, state “mold & mildew” and/or “fungus” are only claimed to be “resistant”. Manufacturers generally disclose that fungi can occur as “biofilm” on these products exterior. Unfortunately, most of these yet unproven composites haven’t stood the test of time & some owners are already starting to have issues.

Newer 2nd generation PVC “CAPPED” composite with recycled wood & plastic core. Is that black mold growing under the “CAP”? If so, we don’t believe any cleaner, including Corte Clean®, can get under the PVC “CAP” to clean the mold stains growing in the wood & plastic core. Is moisture getting in from the end cuts & screw holes, watering the fungi allowing it to grow? Will this lead to de-lamination, especially in freeze thaw environments?


Unfortunately most composite owners do not read composite literature before purchasing and installing an expensive composites, which generally state to clean at LEAST twice yearly (2X), Spring & Fall, or when stains appear, so they are unaware. Rather, composite owners choose to believe claims that composites are absolutely “NO MAINTENANCE” and will never need to be cleaned often propelled by the uneducated at retailers or installers. Unfortunately these composite owners don’t find out that most composites get dirty, with all kinds of common stains, including those from common black mold stains and other common stains until they shows up. They often have no idea what it is?

Picture of a ChoiceDek® Plus® with black mold stains rooting into the recycled PE plastic & Oak wood fibers from which it was made.

Common mold stains generally start out as black, oily looking, spots on composites when it appears. The below photo is what mold stains look like after mold spores (seeds) land on composites, receive moisture, grow and root into composites. If properly proactively Corte*Cleaned®, before the composites receive moisture, hatch, and root into the composites, it generally takes a lot less labor/time and Corte*Clean® (expense/money) to maintain a composites  clean appearance.

BEFORE – New Oily Mold Stains On ChoiceDek®
AFTER – Corte-Cleaned ChoiceDek®
AFTER – Corte-Cleaned ChoiceDek®

If the spores are removed from the composite, prior to the composite receiving moisture, mold stains can not generally occur. Proactively routinely using Corte*Clean®, at least twice yearly, Spring & Fall, at the reduced strength, prior to the composites receiving moisture is the best way to insure composites remain clean, even if the composite looks clean.

Failure to proactively Corte*Clean® composites generally results black mold stained composites, when the composites receive moisture, if mold spores are in the air. The more routine moisture mold receives, the deeper the mold roots, hyphae, that grow in a mycelium, grow into most composites. They attach themselves to the dead organic materials, from which most composites are partially made, such as the wood fiber and feed from the tannins. Failure to thoroughly remove the mold stains and wood fiber tannins, known as “tannin bleeding” from composites generally results in the return of fungus stains sooner rather than later. This costs more time and money, as resilient spores likely remain.

It generally takes (2) “REPEAT” applications of Corte*Clean® at “MAXIMUM STRENGTH” applied to properly prepped, “THOROUGHLY DRY” composites to thoroughly clean mold stains or other difficult to remove stains, when first noticed, and remove the tannin from the wood fiber.

Mold Stained Dry Trex Deck

Thoroughly Dry Mold Stained Trex®

The above photo is a before photo of what mold stains look like when composites have been allowed to “thoroughly dry.” Notice how the mold stains DO NOT appear black & oily. When mold stains are allowed to “thoroughly dry”, it is generally much easier to thoroughly Corte*Clean®. When the mold stains dry out and shrink, it easier for the Corte*Clean® to remove them.

Mold Stained Trex Deck

Corte-Cleaned Trex® Deck

Corte*Clean® works best in removing mold stains when first noticed and the composite has been allowed to “thoroughly dry” generally costing less time and Corte*Clean®. The above picture of a Trex® deck, took one application of Corte*Clean® at 4 Scoops of Corte*Clean® per gallon of water. This is all that is generally needed to clean composites in this condition


Mold stained composite that has not been allowed to thoroughly dry

Oily Mold Stained Composite Not Thoroughly Dry

The above picture is a heavily mold stained composite that is not been allowed to “thoroughly dry.” One can tell it is still wet because the mold stains appear oily, like they just dripped out of a leaky vehicle motor. Failure to let the composite “thoroughly dry” before using Corte*Clean® is not recommended. Always allow composites to “thoroughly dry” before applying or re-applying Corte*Clean®. We can’t stress this enough! Failure to properly use Corte*Clean® to thoroughly remove mold stains or other fungus stains is a waste of time & money. It will likely result in the stains immediately returning when the composites receives moisture.

Trex® deck partially Corte*Cleaned® DRY
Trex® deck partially Corte*Cleaned® WET, stains are still visable.

The above Trex® deck only had one application of Corte*Clean® at maximum strength on the partially cleaned left side of the picture. This deck definitely needs another repeat application of Corte*Clean®, once allowed to thoroughly dry. Look carefully, the mold stains still exist. Water magnifies mold stains. If you can still see them, specifically when the composite is wet, with water, the Corte*Clean® application process must be repeated.

Extremely Neglected Composites

The following photo, below, is what an extremely neglected composite looks like after mold and green lichen or moss (commonly confused with algae, which only generally grows in water, such as ponds) has been allowed to exacerbate itself with the help of routine moisture. Unfortunately the owner of the pictured composite has made the unsightly problem worse by neglecting to properly clean this composite deck for years and by using sodium hypochlorite based products. Generally, lichen or moss is the result of neglecting to properly routinely Corte*Clean® composites at least twice yearly, spring & fall.

ChoiceDek Mold Stains Stairs

Heavily Neglected Fungus Stained Correct Deck®

This Correct Deck® took several additional “repeat” applications of Corte*Clean® at “maximum strength” to get thoroughly clean. Additionally, this deck required heavy brushing and more abrasion. If more abrasion & strength is needed, when brushing, to remove surface debris and build up, as was the case when cleaning this composite, sprinkle Corte*Clean® powder directly onto the composite already wet with Corte*Clean®. Then scrub with a brush until the Corte*Clean® becomes dirty/discolored/heavily saturated with whatever it is cleaning from the composites. Rinse with water.

Corte Cleaned ChoiceDek Composite Deck Stairs

Corte Cleaned!

The amount of time Corte*Clean® is allowed to work is not the key to success. You want to allow it to work until it turns dirty, discolored or heavily saturated or 20+/- minutes. Generally 3 to  7 minutes in direct sunlight is all that is needed for first application. Do not  allow dirty Corte*Clean® to dry and re-settle the stains into composites.  Then it is time to rinse, with water. Water magnifies stains on composites. If you can see stains, you need to repeat the application process to a thoroughly dry composite surface until the stains are thoroughly removed.

Extremely Neglected Mold Stained Correct Deck®
“Repeat” application needed!!!

If the composites you intend to Corte*Clean® has been neglected, power washed or other cleaning products have been previously used, that have exacerbate the mold stains, it may take several additional “repeat” applications of Corte*Clean® to thoroughly clean. Failure to thoroughly clean composites and items on, attached to or above or around composites mold stains is a waste of time. Examples of these items are furniture, flower pots, umbrellas, siding, roofing, concrete, bricks, etc. If the stains aren’t thoroughly removed from these items, they will likely immediately re-occur, wasting time & money. If you do not plan to thoroughly Corte*Clean® composites and surrounding items, expect the stains to re-appear sooner rather than later. If you can still see mold stains, as on the above composite, and the composite is not thoroughly Corte*Cleaned, they will generally, almost immediately return, when the composite receives moisture.  The above composite photo needs another application of Corte*Clean® to thoroughly remove the noticeable green stains.

What Mold Looks Like on Wet and Dry Composites…

Trex Deck - Dry - with Mold Stains

Trex Deck – Dry – with Mold Stains

The above dry TREX® deck looks fairly clean. Don’t be fooled!!! Mold stains still remain in all boards except the lower half of the center board (in between the two white shoes) which was Corte*Cleaned.

Trex Deck - Wet - with Mold Stains

Trex Deck – Wet – with Mold Stains

The above photo shows how water magnifies mold stains in composites. Look closley! If the composite is wet, with water, and mold stains are still noticeable, one must “repeat” the Corte*Clean® application process. Make sure the composite has been allowed to “thoroughly dry” before re-applying Corte*Clean®! One can see, in this picture where Corte*Clean® has not been used, except on the one center board, around the nail in the lower half of the photo.



Corte LLC does not claim that Corte*Clean® “PREVENTS” Mold & Mildew stains from re-occurring. Corte LLC has not found any product or products that prevent mold and mildew or other fungus stains from re-occurring on composites. Most only delay re-growth until they are rinsed from the surface. They are not worth the additional money to purchase and time/hassle to apply.

Corte LLC does NOT recommend the use of sealers! Unfortunately, some customers do not thoroughly clean mold stains from composites. Sealers can actually seal the mold & moisture into the composite and can exacerbate mold staining problems. Sealers have been found to make Corte*Cleaning® composites take more time and cost more money in the future.

Corte LLC does not  recommend or sell any “Preventative” products. Most immediately rinse from the plastic and are a waste of time and money. This is why they are sold in plastic containers, from which recycled composites are commonly made.

Corte LLC and our customers have found that it is best to Corte*Clean® composites at least twice per year, Spring & Fall, before composites receive moisture. Composites loacted in areas known for “mold spores” and routine moister should be proactively Corte*Cleaned more often. Example; Spring, Summer, Fall & Winter. This costs far less time and money to keep a composite clean. Put your composites on the right Routine Maintenance Program for your composites environment.

Are you  aware that most composite manufacturers recommend proactively cleaning composites at least twice (2X) yearly, Spring & Fall or when stains  appear. Corte*Clean® was/is designed and intended to be used “at least twice yearly, spring & fall, or when stains appear” from the time composites are newly installed as its label directions state. It is best to proactively use Corte*Clean® before composites receive moisture and stains appear or deeply saturate into porous composites, at generally the reduced strength, should one decide to generally spend generally LESS TIME & MONEY keeping composites clean. Proactively routinely Corte Cleaning before one puts out the patio furniture in the spring and when one puts away patio furniture in the fall, or when stains are first noticed is highlyrecommended, once thoroughly Corte Cleaned. If composites are located in areasknown for “mold spores” and receive routine moisture, as predicted by The Weather Channel®, or that receive routine moisture from rain, morning dew, high humidity, snow, ice, etc., Corte LLC recommends proactively cleaning composites more often. EXAMPLE: Spring, Summer,Fall & Winter. If you notice mold stains re-occurring, or allow other stains, such as grease & oil, rust, etc. to deeply saturate, you are waiting too long to Corte*Clean® composites and it will likely cost more time and money to clean in the future!

When   proactively using Corte Clean, preferably before composites receive moisture and stains occur or saturate, Corte*Clean® should be used at the “up to 2.5 scoops for routine cleaning” per “1 gallon of water” strength saving money. One can apply Corte*Clean® proactively in any condition, day or night as long as the composites are “thoroughly dry.” This is why the Corte*Clean®label directions state “When Corte*Clean® no longer turns dirty or discolored, it can be diluted & allowed to dry on weathered composite surfaces. Corte*Clean® is designed to clean any time it receives  moisture. A white residue may remain until thoroughly rinsed. Thoroughly rinse composites prior to using.” The best way is to “thoroughly rinse” is to apply before it rains or snows and let the natural moisture rinse it saving water, time and labor. While proactively Corte Cleaning at the reduced strength doesn’t always have that instant cleaning effect, that Corte*Clean® is generally  known for, generally, the composites appear thoroughly clean after they receive moisture.


Proactively  Corte Cleaning generally prevents deep stains that generally cost more time and money to thoroughly clean. This is because the Corte*Clean® soaks into  the pours and wood fibers of most composites. Every time it receives moisture, it releases oxygen, lifting stains until thoroughly rinsed. It is much easier for the Corte*Clean® to lift stains from the bottom up rather than the top down. Proactively applying Corte*Clean® before stains occur  ensures this.

Trex® where Corte*Clean® has been proactively applied but not thoroughly rinsed.

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  1. I used Corte Clean and my brown composite deck and it came out great. However; where my patio table sat, underneath there is a white stain that never came out. It’s pretty noticeable. Any suggestions on what I should do to get rid of the stain.

  2. I had a robber-backed mat on my deck. Now I have white marks on the deck where the edges of the mat was. Is this able to be removed?

  3. Generally, yes, however Corte Clean can speed up the inevitable bleaching/fading or what is referred to as weathering of composites, especially those exposed to sunlight, when Corte Clean is used at higher strengths. Please see the Corte Clean website for further information on weathering and call if you have any further questions.

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