Rendering a wall is not just for adding an extra layer of decoration. Although a properly rendered wall provides a flat, or even a textured, surface on which to form the basis of a room's decoration, it also provides some much-needed protection for the wall itself. Rendering buildings adds weatherproofing, making the walls better able to stand up to extremes in temperature and also the ability to withstand severe rain. Knowing how to render a wall is the key to longer-lasting building work.
Clean the wall to be rendered using a scrubbing brush and a bucket of water. If the render is to go on smoothly, it is important to remove dirt, debris, and any other material such as plant life or even the remains of older renderings. Once the wall is clear, apply a coat of lime and aggregate with a harling trowel. Allow the wall to cure for four days.
Mix an undercoat of one part cement, four-and-a-half parts sand and half a part of lime, if the undercoat is to be applied to a nonporous surface; otherwise use a mixture of one part cement, one part lime and six parts sand. Add water only to the point that the mixture is stiff but workable. Dampen the wall with a sponge and bucket of water.
Apply the undercoat to the dampened wall in upward-moving arcs. Start at the top of the wall and work down, applying thin coats as evenly as possible. Allow two days for the wall to dry. Score the surface of the undercoat with a wooden float, to give a keyed surface to apply the next coat to. Mix a second coat of render in the same proportions as the first. Apply the new rendering. Allow one week for the wall to dry and fully cure.
Consider using battens when applying the undercoat. Nailing the battens vertically into the wall 3 feet apart from one another helps to strengthen the render, making it less prone to movement and cracking. These battens will also allow the rendering to better smooth out the wall, as the extra strength will help support extra render used to fill in distortions in the face of the wall.
When scouring the undercoat, do not press too hard and do not scour the same area too often. If this occurs, the undercoat can be worn away in places, leaving the topcoat to be plastered directly onto the aggregate coat beneath. This will result in some areas of the render effectively having one coat only, which reduces its strength and durability.