Grinding cement screed, yes or no?

Some time ago, I received a cement flow screed, which is now dry and ready for covering. The surface is sufficiently smooth and it does not cup at all.

Back then, the screed layer told me that I did not necessarily have to sand it. Sanding would only be necessary if time was tight and it would dry better as a result.

Out of curiosity, I painted some tile adhesive, as well as primer for the shower waterproofing, and also the paintable sealing paint on the floor without primer.

The tile adhesive sticks perfectly, but the shower waterproofing (Ardex S1-K) can also be peeled off without any problems like a foil with the primer offered by the manufacturer (Ardex P51).

I cannot detect any cement slurry. Even when I scratch the screed, no slurry layer is visible.

Does a cement flow screed now have to be sanded?

The answer to your question can be found in the product data sheet of the manufacturer. Also perform a water droplet test: If the droplet remains on the screed surface for more than 5 seconds without penetrating the surface, grinding should be done, because then there is a likelihood of accumulations of additives, plasticizers, and other substances on the surface. This has absolutely nothing to do with the "Schlämpeschicht," which does not exist in cement-based self-leveling screeds! If a surface is not absorbent (then the water droplet quickly penetrates with a ring formation), the subsequent layers including the primer will very likely not adhere sufficiently. And whether the surface tensile strength is sufficient cannot be determined with "household methods" such as coatings or adherence of tile adhesive. This is where the possible area of responsibility of all DIYers ends and real expertise is required! Regards: KlaRa

I did this test - the drop soaks in immediately.

However, your statement contradicts itself a bit:

If a surface is not absorbent (then the water drop quickly soaks in with ring formation)

I had suspected that the screed absorbs too much and therefore primed it with the appropriate primer.

I had just done the test with the tile adhesive and the sealing as a general test. However, the tile adhesive was already very difficult to remove and was only laboriously removable with the scraper. An adhesive residue still remained on the screed even then. I am studying mechanical engineering and from my experience, I would really claim that the tensile strength, regarding the tile adhesive test, is absolutely sufficient.

@ "restless": "If a surface is not absorbent (then the water droplet penetrates quickly with ring formation)." Thank you very much for the hint. I really made a typo there, did not pay attention. The correct version is: If the droplet penetrates quickly, the necessary absorbency of the substrate is of course present. Regards: KlaRa