TRUE GRIT
Do anhydrite screeds always need sanding? Alan Jackson,
Commercial Manager Gypsol for the UK and Ireland, dispels some common myths
surrounding screed abrasion.
There are many floor screed systems in the marketplace and the selection criteria people use to specify their screed of choice will look at many different aspects of performance. Strength, thickness, thermal performance, drying time and many other characteristics all play a part in the specifier’s decision-making process. One thing which is often cited as a reason for selection is whether or not the screed requires sanding after installation. Anhydrite screeds are sometimes rejected due to the inaccurate perception that there is a requirement to sand them. It might surprise you to read that the need for sanding should never be a factor.
Why is this? To put it in its simplest terms ALL screeds, regardless of type, once they have set and just prior to the application of bonded floorcoverings, should be subjected to light mechanical abrasion using a suitable sanding machine with typically a 60grit or medium sandpaper. This could be by using a large industrial floor sander or could just as easily be a small orbital sander, a pole sander or even a hand- held piece of sandpaper. The choice will depend on the size and nature of the floor. The fact that all screeds should be sanded means that there is no logic to selecting a screed on that basis.
So why do we mechanically abrade screeds? The action of applying light mechanical abrasion, or sanding, serves a number of purposes. Firstly, it removes dirt and debris from the screed which might have been left in place during the construction process. It also produces a key to the screed surface to allow primers and adhesives to more easily adhere to the screed surface. If the screed has produced a loose, friable particulate layer or laitance on its surface then sanding will also easily remove this.
32 | SUBFLOOR PREPARATION
Mechanical abrasion is a simple operation using simple machines but it is often confused with scabbling, grinding and scarifying, all of which are aggressive techniques reserved for applications other than abrading the surface of a screed. They are definitely not appropriate to use on anhydrite screeds.
Installers of finished floorcoverings are sometimes concerned about anhydrite screeds and occasionally go to great lengths to remove inappropriate quantities of the screed surface. There can be a propensity to over-sand, or even grind the screed, to provide a very rough surface or in an attempt to expose coarse aggregate, in the mistaken belief that this will help bond strengths for final floorcoverings.
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