ᐅ Rust stains or similar issues despite using insulating spray

Hello dear forum members,

I’m reaching out again with a curious problem we’re currently experiencing. We had our house built by Rostowbau. They have the walls manufactured by the company Tinglev as aerated concrete blocks, producing the walls as apparent bare concrete elements.

We are close to completion and already progressed with the painting work. The wall was primed and then smoothed over with machine-applied plaster.

Now the problem:

After plastering and the first sanding, dark rust-like stains appeared everywhere. An employee from the wall manufacturer was on site and, without much explanation or concern, recommended white sealing spray and even provided some. We carefully sprayed the stains generously with this spray.

The painters continued their work and have now installed the painter’s fleece. However, the stains still show through the sealing spray. We are currently at our wit’s end. It also seems like the stains are beneath the fleece, but I can’t say that with 100% certainty.

The painters now want to attempt the first coat of paint but are unsure whether the stains will come through.

Does anyone have any idea what these stains might be and why the sealing spray isn’t helping, even though it should seal against almost everything?

Is there any alternative to this spray?

Thank you for all helpful suggestions, and I wish everyone a sunny weekend.