Oxalic vs cream coat

Decided to spot clean some rust spots off a concrete patio today and was in for a nice surprise when I noticed my oxalic acid looks to have etched the concrete. I mixed it 4 oz to 1/2 gallon. image

The other areas don’t really look clean. Did you wash the whole driveway?

Yes, what you’re seeing is red clay staining that isn’t very visible when the concrete is dry. I ended up using oxalic on the rest of it and it cleaned up decent. Some of the spots (especially where I agitated with a brush) like this one appear to have etched the concrete. I didn’t think oxalic could damage concrete like this.

Send pictures of the final product of able. I’d like to see how Ox does with those stains

I don’t think it did etch it. I think those stains where the problem. I think you did the right thing tho. If you use oxalic on one spot, it’s best to do the whole thing. Even if the concrete is pretty clean it will make a difference.

Darn, what kind of brush did you use? The Ox probably didn’t, but your brush sure looks like it. The reason for a brush is to to just spread it evenly and to help it penetrate, not to actually scrub the surface off, lol.

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Haha, I didn’t brush that hard! It’s the same brush I use on my car, very soft bristles! Let this be a warning because it can happen even without brushing! I had some spots where just the droplets had the same effect. It always pays to start with a weaker solution and gradually strengthen.

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It kinda looks like you transferred color from the bristles, any chance?

Looking at the photo it looks like the oxalic was actually working while just dwelling there, that black stain is more than likely oxalic acid reacting with the bristles of your brush. What did you use the brush for prior to going to town on the concrete?