Applying a scratch coat to ensure a stable and uniform background with a good mechanical key is the first step towards constructing a technically sound installation whether that is for a traditional render or for Wallcrete. It is important to ensure that any existing background is structurally sound and clean prior to applying a scratch coat.
The process is simple, as indeed it is for a traditional scratch coat. Thoroughly wet the background to ensure that the wall is fully saturated and then allow the surface to dry to produce a damp surface without any free water. The primer mixture consists of 1 part Bonding Agent and 2 parts cement. These components are measured out in appropriate quantities and then mixed thoroughly with a drill and paddle type set up to produce a uniform consistency. This is then applied to the damp wall, preferably working it in by brush as it really gets the primer into all the nooks and crannies, however many people will use a roller to apply. Whatever the method, it is important that the background is correctly primed.
Once an area has been primed, it is essential that the Wallcrete scratch coat is applied prior the primed surface becoming dry. NEVER allow the primed surface to dry, or it will act as a potential debond. If it is allowed to dry, the area must be primed again.
To produce the scratch coat material, mix one bag of Wallcrete with circa 4 litres of previously diluted Bonding Agent (1 part neat Bonding Agent to 4 parts water). Put the 4 litres of diluted Bonding Agent into the mixing bucket and then add the Wallcrete powder to the liquid, whilst mixing with a (low speed high torque) plasterers mixer. Apply a tight skim coat into the freshly primed wall, and when fully satisfied this has been pushed into the profile of the wall, build up to form a finished layer of circa 3mm. Allow the Wallcrete time to reach a suitable degree of set, then scratch using a notched trowel or scratching tool, to produce a profiled surface ready for application of Wallcrete to thicknesses of 50mm if required.
Allow the freshly applied scratch coat to set and form a good bond with the existing background prior to applying thicker coats to print the pattern.
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Next video is a bit more interesting! Creating a stone effect coping stone!