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Old 06-05-2012, 07:59 PM   #1
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New Cedar Deck Mold/Mildew Stains?


I just laid down my new cedar decking on all new construction (above ground 5') in the last two weeks. We had our first heavy rain last night (had a couple sprinkles the past 2 weeks) and I see small dark spots on all the cedar boards and in some places, heavy staining. I cant believe it could be mold or meldew yet... can it? This is a dry and cool climate (Calgary, AB) and this suddenly shows up on NEW material?? Any ideas on what it might be and /or how to get rid of it?
I have attached pics and would appreciate any advice ! Thanks.
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New Cedar Deck Mold/Mildew Stains?-img-20120605-00053.jpg   New Cedar Deck Mold/Mildew Stains?-img-20120605-00051.jpg   New Cedar Deck Mold/Mildew Stains?-img-20120605-00052.jpg  

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Old 06-06-2012, 07:17 AM   #2
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New Cedar Deck Mold/Mildew Stains?


Tha may just be natural oils in the cedar coming out. But spray with a 10:1 water:bleach mix, and let sit for at least 15 min, then flush with a hose or a low pressure power washer. The problem should be gone. When dry seal with a clear or tinted stain

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Old 06-06-2012, 09:48 AM   #3
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New Cedar Deck Mold/Mildew Stains?


Mine did it too, but I made the mistake of using a water-based stain when I first built it so I washed/scrubbed/sanded it off last year and used an oil-based one.

Not sure if it's related or not - what did you seal it with?
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Old 06-06-2012, 10:18 AM   #4
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New Cedar Deck Mold/Mildew Stains?


Didn't happen to spread a granulated fertilizer or other lawn chemicals recently did you? That almost looks like iron particles from something like iron sulfate.
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Old 06-06-2012, 01:05 PM   #5
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New Cedar Deck Mold/Mildew Stains?


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Originally Posted by sdsester View Post
Didn't happen to spread a granulated fertilizer or other lawn chemicals recently did you? That almost looks like iron particles from something like iron sulfate.
Iron stains was my first thought, just couldn't figure how it could have got there like that, never thought about fertilizer.

Cedar just doesn't do that in two weeks without help.
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Old 06-06-2012, 01:17 PM   #6
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New Cedar Deck Mold/Mildew Stains?


Could be other metals too if fertilizer is the culprit. It is a little known fact that chemical companies can bury just about anything beyond the guaranteed nutrients in sacks of fertilizer. High tech wastes get ground up and stuck in granulated fertilizers, for example under the total lack of regulation of inert ingredients. This is why things like chromium and cobalt are showing up where they could never be naturally---midwest for instance.

Almost started a high-tech junk company once. Among other things, sputtering targets for coating hard drives are stacked up by the warehouses full in California. Since the metals no longer have strategic value the cost of smelting them down and recovering them does not make sense so they get ground up and disposed of clandestinely.
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Old 06-06-2012, 02:37 PM   #7
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New Cedar Deck Mold/Mildew Stains?


Clean with sodium percarb and let dry then seal with an oil based sealer.
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Old 06-07-2012, 09:30 AM   #8
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New Cedar Deck Mold/Mildew Stains?


Thanks all for the input.... spoke with my lumber supplier as well and he feels something possibly blew in on it (ie Pine Pollen) as it has happened on his deck. He suggested not to worry just yet, let it dry and bleach in the sun then wash with TSP and lightly powerwash... should look good as new. Will wait a few weeks then and keep you posted.
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Old 06-12-2012, 11:27 PM   #9
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New Cedar Deck Mold/Mildew Stains?


We see this all the time, It's caused from steel filings coming in contact with the wet cedar, metal filings from the balusters, steel wool, and some shingles contain iron granules too. If you think about it you'll remember the source. All is not lost, this staining can be removed by using a deck cleaner that contains Oxalic Acid. Superdeck makes one, in their brightener product line but I'm sure others do too.

You can quickly create this same staining effect by rolling a piece of steel wool between your hands over a scrap piece of wet cedar and make it faster and darker by adding some vinegar to the mix. Some craftsman do this intentionally to darken wood it's called "ebonizing".

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Old 10-02-2013, 09:12 AM   #10
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New Cedar Deck Mold/Mildew Stains?


Hi, I have the same problem. The cedar deck was installed in June and started to show dark spots that would not sweep, wash or scrub off. It looked somewhat like dirt. I read that it might have been mold. It was sealed with a clear sealer when it was built. 3 months later I thought maybe the seal was inferior and used Cabot deck cleaner, scrubbed and even sanded back to fresh wood. I stained (used toner and seal in one). 1 week passed and it looked great! After the recent rain it looks much like your picture, but under the stain.

The stain company said that lately cedar has been more wet than usual due to rain in some regions and that the mold spores are buried deep in the wood. In addition, some of the wood has what appears to be rotting occuring.

Have you resolved your problem and if so, what did you do? I feel that my wood is defective, but the contractor said that is impossible.

Any feedback would be greatly appreciated.
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Old 10-09-2013, 07:46 AM   #11
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New Cedar Deck Mold/Mildew Stains?


Im in the process of laying my cedar down and worried about this. Especially that most stain mfg's (the good stains) recommend waiting 3-6 months before staining. I think I will need to clean and brighten in the spring as you will but boy that is ugly!
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Old 08-18-2014, 03:25 PM   #12
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New Cedar Deck Mold/Mildew Stains?


I had a brand new cedar deck put on the front of my home - last night and this morning I walked out and it looks like someone threw a bug let of mud on it?? I wanted to put waterseal on right away, but I am worried that it will worsen the problem. How could this happen on brand new cedar just bought from a major home store less than a week ago. We did have a good rainfall (about 20 minutes) while it was being put it, but no other rain. Any suggestions on how to clean it up and should I put the water seal right away. Live in southwest Saskatchewan and have the same climate as Calgary. Thanks.
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Old 08-18-2014, 10:21 PM   #13
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New Cedar Deck Mold/Mildew Stains?


Pressure wash it. Comes out pretty good. I use Thompson water seal every couple of years. Goes on fast and easy.

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