Removal of laitance used to be suggested at 4 days but realistically this is not possible. No manufacturer has ever said before two weeks...I have no idea where that has come from. The issue at this stage is. That the screed is still too wet so clogs up sanding disks etc. we no longer stipulate a time scale as the reality is that as long s it is done before tiling that's fine. We used to say ufh baked it onto the screed and the longer it is left the harder it gets (I have said so myself in the past under the guidance of "better brains") but again the reality is we cannot say either for sure as no testing has been done to qualify it. Even the idea of laitance preventing drying is in reality not specifically tested and our own test results indicate this is probably marginal. That's why we don't put a timescale on it.
Simple fact is if it's hard to remove, it was most likely always hard to remove and if it is stuck like wotsit and will not come off whatever then it probably doesn't need to come off. The surface tensile strength appropriate for most primers is 1kN/m2 so this can be tested if need be. The screed needs sanding not only to remove laitance but to remove other extraneous debris that has the potential to affect primer penetration and adhesion. We view laitance removal as a happy side effect of correct preparation. Sanding should really be carried out with all screed types, not just anhydrite. (*waits for the I never sand sand cement screeds furore)
