Some time ago, I had a cement-based self-leveling screed installed, which is now dry and ready for covering. The surface is sufficiently smooth and does not cup at all.
At the time, the screed installer told me that sanding was not necessarily required. Sanding would only be needed if time was tight, to help it dry faster.
Out of curiosity, I applied some tile adhesive, as well as the primer for the shower waterproofing, and also the brush-on waterproofing coating directly onto the floor without any primer.
The tile adhesive adheres very well; however, the shower waterproofing (Ardex S1-K) can be peeled off like a membrane without any issues, even when applied over the manufacturer’s recommended primer (Ardex P51).
I cannot detect any cement slurry on the surface. Even when scratching the screed, no slurry layer is visible.
Is sanding necessary for a cement-based self-leveling screed?
At the time, the screed installer told me that sanding was not necessarily required. Sanding would only be needed if time was tight, to help it dry faster.
Out of curiosity, I applied some tile adhesive, as well as the primer for the shower waterproofing, and also the brush-on waterproofing coating directly onto the floor without any primer.
The tile adhesive adheres very well; however, the shower waterproofing (Ardex S1-K) can be peeled off like a membrane without any issues, even when applied over the manufacturer’s recommended primer (Ardex P51).
I cannot detect any cement slurry on the surface. Even when scratching the screed, no slurry layer is visible.
Is sanding necessary for a cement-based self-leveling screed?
You can find the answer to your question in the manufacturer’s product data sheet.
Also, perform a water droplet test: if the droplet remains on the surface of the screed for more than 5 seconds without being absorbed, the surface should be sanded, because this indicates a likely accumulation of additives, plasticizers, and other substances on the surface. This has nothing to do with a “weak surface layer”; such a layer does not exist in cement-based flow screeds. If the surface is not absorbent (then the water droplet is quickly absorbed with formation of a ring), the subsequent layers, including the primer, are very unlikely to adhere properly.
Whether the surface tensile strength is sufficient cannot reliably be determined with simple household methods such as brush tests or adhesion of tile adhesive. At this point, the possible responsibility of DIYers ends and true professional expertise is required!
Regards, KlaRa
Also, perform a water droplet test: if the droplet remains on the surface of the screed for more than 5 seconds without being absorbed, the surface should be sanded, because this indicates a likely accumulation of additives, plasticizers, and other substances on the surface. This has nothing to do with a “weak surface layer”; such a layer does not exist in cement-based flow screeds. If the surface is not absorbent (then the water droplet is quickly absorbed with formation of a ring), the subsequent layers, including the primer, are very unlikely to adhere properly.
Whether the surface tensile strength is sufficient cannot reliably be determined with simple household methods such as brush tests or adhesion of tile adhesive. At this point, the possible responsibility of DIYers ends and true professional expertise is required!
Regards, KlaRa
I performed this test – the drop is absorbed immediately.
However, your statement contradicts itself somewhat:
I suspected that the screed was too absorbent, so I primed it with the appropriate primer.
I carried out the test with the tile adhesive and waterproofing as a general check. However, the tile adhesive was already very difficult to remove, and even with a scraper, it was a laborious process. Some adhesive residue still remained on the screed. I study mechanical engineering and, based on my experience, I would truly say that the tensile strength regarding the tile adhesive test is absolutely sufficient.
However, your statement contradicts itself somewhat:
KlaRa schrieb:
If a surface is not absorbent (then the water drop soaks in quickly with ring formation)
I suspected that the screed was too absorbent, so I primed it with the appropriate primer.
I carried out the test with the tile adhesive and waterproofing as a general check. However, the tile adhesive was already very difficult to remove, and even with a scraper, it was a laborious process. Some adhesive residue still remained on the screed. I study mechanical engineering and, based on my experience, I would truly say that the tensile strength regarding the tile adhesive test is absolutely sufficient.
@ "rastlos":
"If a surface is not absorbent (then the water droplet quickly penetrates with corona formation)."
Thank you very much for the note.
I actually made a typo and wasn’t paying attention. The correct version is:
If the droplet penetrates quickly, the necessary absorbency of the substrate is clearly present.
Regards, KlaRa
"If a surface is not absorbent (then the water droplet quickly penetrates with corona formation)."
Thank you very much for the note.
I actually made a typo and wasn’t paying attention. The correct version is:
If the droplet penetrates quickly, the necessary absorbency of the substrate is clearly present.
Regards, KlaRa
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