Trex/composite decking

I’ve done a few composite decks with green mildew or the black spots and I’ve found that the only thing that has worked for me is straight 12.5% sh in a pump up sprayer and rinsing with a wide angle tip. Unfortunately that takes forever. I don’t have a soft wash pump so I can’t apply it that way. I usually use a xjet for houses. Has anyone had success with any of the xjet proportioners with a strong mix to clean composite decks?

Also I’m worried about damaging the deck using straight 12.5%.

I have been using a pump up for years on decks. I finally got a dedicated pump and still usee my pump up more. The dedicated pump puts out 5 gpm which is great but it’s almost too much. I go through three times as much SH but I save a lot of time. As for your question, I don’t think there is a better way as far as chemicala go.

Thanks @Clean_Blue. What mixture do you use?

Anyone else using an xjet or is the pump up sprayer still the best bet?

“Composite” decking is typically ~ 50% cellulose (wood fiber). Some decking products are up to 70% in cellulose content.

Straight 12% SH will quickly destroy lignin, the “cement” that keeps wood fiber structurally intact. 1.5 or at most, a 2.5% SH solution will kill any active mold/mildew without appreciable damage.

Aside from artillery fungus, any “spots” left are stains. Try adding some soap and surfactant to your mix. Keep wet and scrub the bad areas with a brush. For spot treatment, I believe there are some products on the market but have never used them.

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Rick Petry is right on with his advice! I have done a few of these where
I am. Most of them were the original trex decks, the ones that started a
big lawsuit for Trex. This worked for us - 2% sodium hypoclorite with the
Simple Cherry from Bob as soap surfactant. Applied with pump sprayer
and allowed to dwell 20 minutes, making sure to keep wet. While waiting
for dwell time , scrubbed each board with a deck brush. After dwell and a
scrubbing rinsed off with 640 psi walking each board at a time. They turned
out pretty well, and the customer had never cleaned them.

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I use very little surfactant or else I spend too much time trying to rinse the suds away. I frequently use a brush on the difficult spots. That way I don’t have to resort to high pressure which can strip color or make stripes all over the deck.

So to get to 2.5% SH Solution in a 3 gallon pump up sprayer:

Just over a half gallon SH (5/8 or .625) / 3 gallons of water = .208 or 20% of my mix is SH

.208 x .125 (12.5% SH) = .026 or 2.6% SH

Is my math correct here to figure out SH strength?

IMO I don’t think a much stronger mix is the answer- are you letting it soak for long enough to takes its effect on the algae?

I know you guys are right and know what you’re talking about. I did composite decks yesterday and today and am applying what you guys are saying. I have one tomorrow too so this information has been good. Today I tried an xjet with a 5:1 proportioner and my normal house wash mix. The deck wasn’t too dirty and it wasn’t sunny out but I let it dwell for 20 mins and scrubbed each board and then rinsed the length of each board. It turned out well.

Maybe the black blotches and things left behind are tannin stains. I’m going to try some oxalic acid on the one tomorrow after I wash it. Or maybe I’m just being too picky and trying to get it perfect. I’ll try to take some decent pictures.

So here are a couple shots of the deck I did today. As you can see the mildew is gone but what about the black left behind? I did spray some f8 wood brightener at 4oz/gallon of water with a pump up sprayer and it didn’t make much of a difference. It ended up raining so I never saw the deck dry. Any suggestions?

Any suggestions?

I’ve been doing more trex decks lately. I used a 2% mix on this one and hit it with my 1000 psi tip. It’s first gen. When I used the 40 deg 1000 psi tip, I saw it go from grey to brown. It took slow even passes to not stripe it up. I would not have continued for fear of discoloring but I saw that where the mat had been for years it was brown also. I figured that must of been the original color. Hitting it with the medium pressure tip looked the same as when washing top dead layer off wood fences. It came out uniform, all evenly brown, but then it dried back to gray again. It looked good in the end but I’m wondering if I took off more than just dirt and growth? Is there a top layer that’s gone now? Last pic is it dry and gray again (sucks because it’s the only pic I have of it cleaned and dry, was part of the final house wash completion photo, hence you can see only a tiny bit of it but it shows the gray color