It is not necessary to coat brickwork to achieve acceptable levels of serviceability and durability, however render may improve a wall's acoustic and fire performance. Cavity construction as two leaves of brickwork or as brick veneer ensures a waterproof wall. Thus any coating such as render or paint will be for aesthetic reasons, but need similar levels of serviceability and durability as uncoated brickwork.
Render and Bagging
Both are essentially coatings of cement, sand and lime, differing in their thickness and method of application. Bagging is the application of a mortar mix to flush jointed brickwork with a hessian bag and produces a rough texture through which the brick pattern is evident. Render is applied by trowel in one or more coats, depending on the finish and exposure conditions. The maximum thickness of any coat should be 15 mm with final coats kept to 10 mm.
Most bricks provide good adhesion and joints should be well raked. High strength dense render is more liable to shrinkage cracking than weaker mixes. Large areas of continuous rendering should be avoided by providing movement joints in the render and render should not be carried across structural movement joints. Render should not be exposed on horizontal surfaces such as sills or parapets.
The most common paints are cement based and acrylics. Cement based paints have a longer history than acrylics and have a reputation for long term durability and colour fastness, to the extent that at least one manufacturer claims that no recoating will be necessary. In most cases, the brick pattern will be evident and the colours of some paints will fade.
Acrylic based paints range from relatively thin coatings to thicker coats applied by trowel. High-build coatings applied by roller or spray give better performance than thinner coats.
As with all paints, surface preparation is important, as is adherence to the manufacturer's instructions. Brickwork should be free of efflorescence, dirt and dust and the paint applied to dry or to dampened surfaces. All acid washing must be neutralised. New brickwork should be allowed to dry out and six weeks should be allowed for render and bagging to cure and dry before painting.
Paint applied to flush or ironed joints has less risks of cracking than brickwork with raked joints as the paint does not have to "bend" around sharp edges. Some paints can only be applied to flush jointed brickwork.
AS/NZ 2311:2000 "Guide to the painting of buildings" is relevant.