Laitance and sanding
Laitance is a weak, dusty skin of fine particles that can form on the surface of an anhydrite screed as it cures. Left on, it weakens the bond of adhesives and slows the drying. It's taken off by sanding.
On a traditional calcium sulphate mix, brush or sand off loose laitance at 7 to 10 days, then sand again just before the floor goes down, with an orbital sander, a vacuum and a 60 grit disc.
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Which screeds need it
| Screed | Laitance |
|---|---|
| Calcium sulphate, traditional mix | Can form |
| Calcium sulphate, low laitance mix | Additive prevents it |
| Hemihydrate liquid screed | Doesn't form |
| Cement-based liquid screed | Possible residue |
| Cement-based liquid screed | None after curing |
How it's sanded
- WhenFirst pass at 7 to 10 days on a traditional anhydrite mix; final pass immediately before the floor finish.
- With whatAn orbital sanding machine with an integrated vacuum, a 60 grit paper or carborundum disc.
- How muchJust the skin. Sanding removes laitance and dirt and gives a key. It isn't for taking depth off the screed.
- ThenVacuum, then prime with the adhesive maker's primer. Primers and floor finishes.
Why early sanding helps
Taking loose laitance off at 7 to 10 days helps the drying, because the skin holds moisture in. On a floor with a deadline, ask the contractor whether that first pass is in their price, and whether they're pouring a traditional or a low-laitance mix.
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Laitance and sanding questions
What is laitance on screed?
A weak, dusty layer of fines on the surface of a calcium sulphate screed. It's sanded off so adhesives bond and the screed dries.
When should liquid screed be sanded?
On a traditional calcium sulphate mix: a first pass at 7 to 10 days, then again just before the floor finish.
Does every liquid screed need sanding?
No. Cement-based liquid screed has no laitance to sand, and nor does hemihydrate. Cement-based liquid screed is lightly abraded.
What grit is used to sand screed?
60 grit paper or a carborundum disc on an orbital sander with an integrated vacuum.
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