Cedar deck problems - Black Spots

by Dave
(Ontario, Canada )

I had a cedar deck built last year and have had nothing but problems. Right after it was built the wood started turning all spotty black. It looked sort of like mold.

Apparently this was caused by a grinder being used during construction..

After washing the deck with deck wash the black came out but the wood got all white and fuzzy after drying...so had to lightly sand the deck and got some ready seal on it finally the deck looked ok...

Now after this winter areas of the deck are all hairy, like brown strands all coming off the deck. Under these areas the deck looks new. The hair reminds me of cedar bark. Some areas still look ok with the ready seal.

I'm hoping the whole deck doesn't need to be sanded. All I've been doing with this deck is work and not getting to enjoy it.

I'm really starting to regret going cedar. The railing and privacy panels still look good with the ready seal.

What to do?

Could this be just cheap or improperly dried cedar?

Editor's Comments

That is iron filings rusting on your deck. They can be very very fine and they are really tough to remove afterwards. You can really see them and they grip and stick well to the wood fibers.

That was a dumb and inexperienced move by the builder for sure.

What you are seeing on the cedar is the fibers from the sand job. You can try to resand but with a finer less course paper. If you sand too hard or if you pressure wash to hard you end up with fibers being pushed up and it later becomes quite obvious.

This is also a problem cabinet makers face when they are trying to get a piece of wood to be really smooth before finishing.

What they do is use a hand scraper with a tiny burr on the edge and it literally cuts each errant fiber right off. It's beaufitul. But easy for a small cabinet. Not quite as easy for a larger deck.

I really would consider that if you want to fix the problem or else try the lighter sand. That may do it.

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Aug 15, 2015
by: Dave

I would try the deck wash or there are formulas online using oxy clean laundry detergent. This should clean and brighten. I continued to have problems with my deck and I used two products the next year and to me cedar is just way to much work unless you let it grey I guess.

I sold that house and now enjoy a concrete patio that I've never had to do anything to.. I'll never touch cedar again!!!!

Aug 15, 2015
Grey Black Spots On Brand New Deck
by: Mike

We just built a new deck last month and after the first rainfall there's black/grey spots all over! Some areas are extensively worse then others. Particularly where there's still sawdust sitting around.

Is there anything we can do to remove them, other than sanding?

Editor's Comments

Almost certainly whoever built the deck was grinding or sanding metal and fine iron filings have landed all over your deck. Maybe they were from a container of nails or something that had fine iron pieces in it.

But you are looking at metal either rusting or reacting to chemicals in the wood. Since I am not a chemist I cannot offer you a chemical solution.

Dave's comments about a deck cleaner oxy clean might work. I would consider giving it a try.

You either sand it off, right down deep enough that you remove all the fine pieces of metal or you use a scraper just like a cabinet maker would and carefully scrape each board removing a fine layer of fiber and the fine filings with them.

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