Renovating Patch Rendering

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Published: 23rd December 2010
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Rendering is the layer of plaster that covers the brickwork on your building. It is essential to have rendering to protect the brickwork from penetrating damp. Over time, cracks will appear or rendering will flake. These faults need to be repaired. This is the best way to do so.

Filling rendering
Coat with exterior wall paint. Little cracks should be filled, but any repair will likely be obvious until the wall is painted.
1 - When treating cracks, you first need to chip away all loose material and undercut the cracks to make sure the filler is well anchored and will not fall out later. Then dust out the cavity and dampen it with water.
2 - Fill the crack, either with an exterior grade filler or mortar mix. The former is convenient but only economical for small areas. Dry mortar mix can be obtained in small quantities or it is possible to make your own with one part cement, one part hydrated powder lime and six parts sand. You can use a proprietary liquid plasticiser in place of the lime.

Patching rendering
If rendering is coming away from the wall, the cause could be dampness, a fault within the rendering mix or defective brickwork joints. This generally shows up as bulges.
Tap suspect areas lightly with a hammer. If there's a fault, the rendering will fall away and you should clean off the affected area with a bolster chisel and club hammer until you reach rendering that adheres firmly to the wall.
A rendering mix consists of one part cement, one part lime and six parts sand. The lime makes the rendering more flexible and simpler to use. But it must be applied rapidly, since after mixing you have only 15 or 20 minutes until it becomes too firm to work. Don't add water to a setting mix since this will only weaken it.

Technique
For regular house work it is best to apply two coats of rendering. The first ought to be thick and the second a 6mm top coat.
1 - Should you try to dab the mix on the wall it'll just fall off once more. The idea is to apply it with a sweeping motion. Work from the bottom of the patch upwards with the first coat. When this is starting to set, scratch it to form a key for the second coat to grip.
2 - Wait 24 hours before applying the second coat, beginning from the top this time and working from left to right with the exact same flowing movement.
3 - When complete, level off with the surrounding wall surface. Leave this until it is nearly dry, then draw a water-dampened towel across it to give a smooth, flat finish.

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