Where mortar has been used between each brick, you should wait for 4 to 6 weeks to cure properly before applying the paint system. The curing process for mortar is important as it allows the alkalinity content to drop to a level where it won’t affect paint adhesion. It also ensures the moisture content in the mortar falls below 10%, and the time allows efflorescent salts to migrate out of the mortar. There are ‘green render sealer’ products that are available to minimise this curing period, but allowing the mortar to fully cure is recommended for best performance of the paint system.
Oil-based prepcoats should be avoided on masonry surfaces even after curing, as the alkalinity of the surface reacts with the alkyd resin of the paint and affects adhesion.
The Australian Standard AS/NZS Guide to the Painting of Buildings specifies only two coats of a quality acrylic exterior topcoat is required for exterior brick, FC sheet, concrete, masonry, cement render or AAC blocks (Hebel). It also states that additional prepcoats, topcoats or both may be required depending on the substrate colour, texture or porosity. For this reason we recommend a 1 x Prepcoat and 2 x Topcoat system as best practice.