The wall to be rendered should itself be dry, but the surface should be well wetted, to prevent it from absorbing at once all the water in the cement; it should also be sufficiently rough to form a good key for the cement.
Screeds may be formed on the surface, and the cement should, if possible, be filled out the full thickness in one coat, and of uniform substance throughout.
When cement is put on in two or three coats, the coats already applied should on no account be allowed to dry before the succeeding layers are added.
The coats last applied are very liable to peel off under the effects of frost or exposure.
Many of the quick-setting cements, such as those mentioned below as adapted for internal work, are rendered in one thickness of cement and sand, and the face afterwards finished and brought to a surface with neat cement.
Sand may be added with advantage to most cements, to prevent excess in shrinkage and cracking; sometimes a very large proportion is used (see Part III.)
The material best adapted for rendering the external surfaces of walls is Portland cement. Other materials such as Roman cement, are, however, frequently used, but have not the same adherence, appearance, or weathering properties.
1 Seddon's Notis.
The objection to Portland cement, from an economical point of view, is its first cost, the greater labour required in using it as compared with that necessary for other cements, and also the time frequently wasted upon it, for, in consequence of its setting slowly, there is a tendency for the men to go on working it too long.
In order that it may set as quickly as possible, the less heavy varieties should be selected for rendering (see Part III.)
External rendering is often marked with lines, so as to represent blocks of ashlar stone.
Both Portland and Roman cement are mixed with a good proportion of sand for external work.
The Portland cement may be used in the proportion of 1 cement to 3 sand, and the Roman cement with 1 part of sand to 1 of cement for upright work.
For cornices, mouldings, etc., about half the quantity of sand should be used, but some is required to prevent cracking.
Parian, Keene's, Martin's cement, and others of a similar character, are eminently adapted for internal work.
The treatment of the several descriptions varies slightly, but they are generally laid in a thin coating of about 1/8 inch depth on a backing of Portland cement and sand. In some cases the backing is formed of the quick-setting cement itself, mixed with 1 to 1½ of sand.
Most of them can be brought to a beautiful hard polished surface, and are ready to receive paint in a few hours.
Portland cement is also used for internal work often with a very large proportion of sand, as much even as 9 parts of sand to 1 of cement being recommended.