Spots keep showing up on on unfinished cedar boards


Old 10-17-20, 01:12 PM
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Question Spots keep showing up on on unfinished cedar boards

Hello all, I'm working on putting in a new iron railing and reskinning a deck with new cedar. I've started replacing the old fascia boards with cedar, and noticed that after a several days, I'm seeing some significant dark grey spotting on the new cedar.

I've sanded it off once, but it ended up coming back.

I thought it might have been dust from using the angle grinder to cut the iron railing parts, so I started doing the angle grinding farther away to try to eliminate this variable. It doesn't seem to have made a difference.

I'm in a pretty dry climate, although we have had a couple light rain showers over that time period. It is pretty windy here, job is on top of a pretty tall hill. Wondering if something is blowing up onto the boards.

I'm at my wit's end here. Anyone have any suggestions? I don't see how it could be mold. Seems like it has to be rust...

I guess I could sand right before staining, but the whole deck is over 1000sqft. I'd prefer to prevent the problem in the first place rather than all that sanding. Seems crazy to have to do that with new boards.

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Old 10-17-20, 01:32 PM
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Cedar doesn't react well to iron oxide or iron filings, so I'm sure that's what it is. It's why you should always use stainless steel or ceramic coated fasteners on cedar... Hot dipped galvanized is next best thing. But a brite common nail would stain the cedar something terrible. That's what's happening but on a much smaller scale.
Old 10-18-20, 02:26 AM
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I'd clean the wood up and apply a deck stain, that should prevent the steel filings creating a reoccurance.
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