Washing varnish
When producing finely washed surfaces, the cement slurry on the concrete surface is prevented from hardening by contact retarders. After stripping the precast elements, this uncured surface can be washed off with a water jet. In this way, the colored aggregates of the concrete are shown to greater advantage. Basically, two processes are available for exposed aggregate concrete: One is the negative process and the other is the positive process. In the negative process, either a contact retarder is applied directly to the mold, or - especially for smaller workpieces - a paper treated with contact retarder is inserted into the mold. The workpiece is then processed with water and a brush after 16 - 24 hours of stripping in order to wash off the layer not hardened by the retarder. In the positive method, the concrete surface is treated with a contact retarder only after the concrete has been placed, and the surface is also treated with water and brush the following day.