It is very important to sand the surface of a Liquid/flowing screed to remove the surface laitance. If the surface laitance is not removed final floor finish adhesion issues could arise.
In ideal conditions, floor sanding should be carried out 7 days after laying the floor screed but this can be greatly influenced by conditions within the building envelope with damp or humid environments likely to extend this considerably.
It is advantageous to remove the laitance at early stages whilst it remains weak and friable however if left in place a small delay to drying times may result and effective removal may be more difficult to accomplish.
Floor sanding of the surface is generally carried out using an industrial orbital sander fitted with a carborundum disk (typically 60 grit) and suitable dust extraction. Due to the fine nature of the laitance, it may be necessary to use several grades of carborundum to avoid clogging of the finer disks. Strongly adhered laitance may require the use of an abrasive copper disk followed by the use of a finer carborundum disk to produce the desired surface.