Retarder can be used on horizontal & vertical surfaces to retard the set of the surface and allow the aggregate to be exposed. For Horizontal Surfaces, strike the base to the specified level, consolidate the concrete by using a wood or magnesium float and then spray. Retarder at a rate of 100 â 250 square feet per gallon depending on the nature of the aggregate and ambient conditions. 6-12 hours later, pressure wash the surface to remove the retarded cement paste and expose the aggregate. A mechanical brush or light acid wash may be required to finish the surface.