Yep. Sure looks like the hot SH mix basically oxidized something in the concrete.
In the future, you’d do fine to just use something much weaker on your initial application (Krud Kutter, Simple Green, or hell even just household detergent), then power wash, and then (if needed) put on a light SH post-treatment to brighten things up or even out your surface cleaner streaks. In my opinion, your process of using 3-4% SH applied straight away is a giant red flag.
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Pretty common process on dirtier than average concrete as far as I’m aware…
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Not metal work but his wife has had a lot of garden stuff there. I searched that a lot of fertilisers have traces of iron in them. Haven’t had chance to use Oxalic on it yet to see if it will resolve the issue but will be done soon.
I actually did try a weaker sh mix (1.87%) on one my own house backyards but unfortunately it left a lot of the black spots. I will put post treatment to see if that clears it up. If it works they will be my go to. Pictures below. Unfortunately couldn’t stick around for it to fully dry.
Are you surface cleaning and post treating? Most of the time you won’t need a pretreat, just a 1-2% post treat, occasionally a stronger mix for really bad lichens.
Some of those dark spots are the gravel showing thru. You need to learn to clean correctly. I clean drives worse than that all day long and use a hw mix on front and back ends.
Do all the drives there have those funky swirls in them? Have never seen that anywhere.
The swirls are on slot the concrete in my area it’s from the concreters. A lot of the newer places don’t have it.
What kind of dirt do you have there? A lot of concrete here has that look from the clay dirt. It was probably just covered. Use bleach on it.
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Were chemicals left on for a while prior to wash?
Was a resolution found?
