What % SH is safe for wood?

I have a customer, a guy I cleaned his house, is having me clean up a deck at his new house. It’s in rough shape, so his plan is just to clean it, then do a dark solid stain to get some more years out of it. My plan was just to hit it with like 1% HW mix then a good rinse at 1000 psi. I don’t normally do wood, so I’m not that experienced with it, just a bit of reading I’ve done on here. I know too high of mix can fur the wood and if you don’t post treat with acid it will grey out quickly. My question is, what percentage and how much time does it take for that to happen? If I only use 1% and rinse really well, will it still look decent for a month or two before he gets around to staining it?


LOL, that is not in rough shape. That’s like barely aged. 1% will probably work on that and you can go up to about 1200 psi.

This is one in fairly rough shape - actually about average for what I see

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I used 3% on this fence as it had really bad mold, everyone yelled at me for not using oxalic afterward (which I will now) but that and 1300psi came out beautiful. Here it is three months later, no greying.

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Cool beans, this should be an easy one, that’s why I took it. Just wanted to confirm 1%had no real chance of graying without ox.



This was a quick and dirty cleaning - nothing fancy at all which is good cause I don’t do fancy! I sprayed with house wash mix, let sit for about 10 minutes, white tipped it at about 1,000 psi.

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2%, short dwell, 1000psi, rinse, rinse, rinse, ox, rinse, rinse, rinse. This deck had like four layers of different paint at one point and it was all stuck between the boards still so it doesn’t look great but definitely much better and that’s all they were hoping for.

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My question is, if I go higher like 2%, is ox needed or will it look fine for a while until it’s stained? My thinking is just 1% will work and not grey out without following up with ox. I just don’t want to waste time and ox if it isn’t needed. They don’t care about brightening it up because it week get a dark solid stain.

Looks great BTW, nice job

Absolutely use ox when you’re done. Especially if they’re staining. It will help relax the wood fibers to limit furring. That will save a lot of time and labor on sanding before stain.

HW mix probably not strong enough to even cause furring on wood that still has tight grain.

Don’t go higher than 2%.

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It’s hard to see in the pics, but the grain is not that tight anymore. It’s not super dirty, but it is old and breaking down.

Just do as you planned, you’ll be fine as will your customer.

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if you recall, how long did it take you to walk those boards out? That looks like a really long slow walk day.

I’m pretty quick at it. I don’t even remember, probably an hour or so for deck and handrail, of which there was a lot. It was so bad I may have used SC on it, in which case I did the floor in about 15min and spent rest of time on handrail and pickets

Wow, that is pretty quick. It takes me longer to walk the boards. I was on a set of steps with a landing, posts, and spindles that took me over an hour to rinse (strip) the other day. Maybe I’m doing something wrong, used hydroxide dwelled, scuff test, etc, or that stuff cleans up really well and the rinsing is faster.

What psi did you end up using? I bought four 1300psi nozzles to try on some of my decks.

I like 1000-1200 psi for most wood, 40 degree, but that’s just me…

Damn you cleaning two boards at once? I prefer a 25° because it’s a perfect board’s width at 6-8" away.

I use 25 too, but seems like 40 works for me most of the time…