New decking black blemishes Q?

Last summer I replaced the decking on my deck with 5/4 x 6 pressure treated something purchased from a reputable lumber yard. All went well. I did NOT put any type of stain or finish on it 'cause it was pressure treated. Now, some 6+ months later and after the horrendous NE winter we've had, there are black streak marks showing up on many of the boards. Any idea what these might be and more importantly how I "might" be able to get rid of them???
Thanks
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bobmct wrote:

Maybe iron stain from fasteners? I also thought maybe mildew but have no idea if it will grow on PT; one would think not but...
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|there are black streak marks showing up on many | of the boards. | Any idea what these might be and more importantly how I "might" be | able to get rid of them??? |
Probably mildew. A strong bleach solution will make it fade out. Something like 1 or 2 parts bleach to 5 parts water should be about right. The traditional recommendation is bleach and TSP, but the TSP is only for cleaning. It won't affect mildew.
After you do that you might want to consider some kind of finish. Just because it's PT doesn't mean it doesn't need a finish. The first reason would be simply because PT is very ugly. The second is to protect the wood. Like cedar, it will resist water well but it doesn't last as well against drying from sun exposure.
Your options are somewhat limited. It's nearly impossible these days to get a solid, oil-base deck stain. Water base deck stain is junk. It just wears off in a year or less. It also shows dirt badly. Semi transparent oil stain won't look as good over PT because the wood is so ugly and dark in the first place, but it's a pretty good choice. Two or 3 coats of something like cedar tone should almost cover, and is easy to re-apply.
Sherwin Williams has come out with an acrylic product called Super Deck that's supposed to put a very tough finish down, and even fill cracks up to 1/4". It sounds interesting but farfetched to me, but I'm curious. The problem with something like that, though, is that it's unproven, and if it turns out to be a lemon you've ruined your deck.
Another thing I've used when I've wanted a nicer looking paint finish with a rough PT deck is to mix Benj Moore high gloss oil Impervo with liberal amounts of boiled linseed oil and thinner, so that it's a thin, stain concsistency. The result is a glossy stain that provides solid color but doesn't peel, and it holds up pretty well. (The drier the wood, the better it will work.)
I use a similar mix in black for rusty wrought iron railings. The thin blend soaks in nicely and doesn't peel. One caveat: I think they've reformulated HG Impervo since last time I used it. I haven't tried the new version. But the general idea is to just get the best quality gloss, exterior oil paint you can and use that as basically the drier and tint for the linseed oil concoction.
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