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Posted: Mon May 16, 2005 10:27 am
by mattrgee
The rendering on my out house has come off in places and needs patching up. What is the recommended mix for this type of rendering work?
Many thanks
Posted: Mon May 16, 2005 12:47 pm
by danensis
If the outhouse is built with lime mortar, then you need to use a lime render, otherwise you will simply be storing up trouble for the future. You should use one part of semi-hydraulic lime to two parts of sand.
Posted: Mon May 16, 2005 11:40 pm
by Tool
4 to 1 mix is what you want with a touch of waterproofer
Posted: Wed May 18, 2005 2:15 pm
by danensis
No, you don't want any waterproofer. The idea is to let the render breathe.
According to http://www.northcoast.com/~tms/render.html
Lime rendering has been applied in one, two and even three coats, although normally a dubbing out or scratch coat is followed by a finishing coat. The first application, as the name 'dubbing' suggests, is intended to overcome some of the inaccuracies that occur in old masonry walls. This is particularly noticeable in earth structures where quite deep pockets may need to be filled in order to achieve a relatively flat surface. The first coat is usually prepared from one part matured slaked lime putty mixed with three parts of a well-graded aggregate. Rather misleadingly, this is sometimes referred to as ''sharp'' sand. When filling deep pockets and applying a relatively thick coat, the aggregate should contain particles of up to one-fifth of the thickness of any individual coat and should also contain equal quantities of all the sievings through to the fines. Experiments carried out at the Lime Centre demonstrate that a render repaired in this way will dub out well without slump and with very little crazing. On the other hand, a render prepared from badly graded material is more susceptible to slump, and shrinkage crazing will occur during the cure. To reduce the initial suction of a dry wall, the substrate will need to be made wet before the application of each coat.
Unlike modern cement renders, a lime-based material should be applied, ruled off and left for about 24 hours before any attempt is made to float the surface. Working a lime render immediately after application will tend to draw fines to the surface, which will increase the risk of slump and probably cause crazing during the cure.
Carbonation is a slow process and several days often pass before the surface of the first coat is firm enough to take a finishing coat. Full carbonation often takes weeks, or even a year. The floating or finishing coat is usually prepared from one part lime putty to one to three parts well-graded aggregate, but with a smaller maximum particle size. Research shows that renders prepared from finer sands tend to craze during the initial drying-out period and are often reinforced with animal hair. A good technique is to leave the material for about 24 hours, after which it is quite likely that a certain amount of crazing will have taken place. Using either a wooden or a plastic float, the surface should be gently rubbed-up to generate a certain amount of fat before pressing firmly to achieve a finish.
Posted: Wed May 18, 2005 7:22 pm
by mattrgee
Thanks