Yellow stains reappearing from F9 or Oxalic on concrete?

Hi all,
We restored a big and long driveway that hadn’t been washed in years using a f9 (oxalic) acid wash.

It took nearly 3 days of washing since the customer wanted the entire driveway and all hardscape areas done.

When we left, it looked incredible, like brand new concrete… A week later the customer called and sent photos of a yellow stain appearing on the surface (like the acid wasn’t rinsed well enough?) However, after acid washing we rinsed the concrete thoroughly and power washed it well.

We are set to go out there next week to trouble shoot the problem but he is hoping to list his house in a few weeks and doesn’t want the concrete looking subpar.

Any help is greatly appreciated as this one has us scratching our heads a little!

Here are some photos-
I have them labeled: The first 2 are after we were done, the third pic was before we started anything, and the last 2 are of the yellow appearing about 1 week after job was completed

Kris, text these pics to me if you can. 760-774-5306

F9 is different than an oxalic wash. Personally nothing like those pics have ever happened to me.

First thing we need to find out is where did those stains come from? Any idea? If it is a reaction in the concrete coming outward then it’ll be a problem. Hopefully it was topical so we can make it look good again.

Detailed pics will help. I am thinking it may be more “stuff” coming from within the concrete and may only need another light, diluted mist coat of product.

Sent from my iPhone using Tapatalk - now Fr

We have had this happen before as well. We treated again and it turned out great.

It looks like a light pre-treat should do the job. Keeping Craig Harrison in the loop here is the way to go.

Could be the sprinkler system. I did sidewalk down the street and it was white as snow when I left. A week later there were yellow stains everywhere the sprinklers hit.