Plasterer - Methods

Methods

For fine plasterer's sand-work, special sands, not hitherto referred to, are used, such as silver sand or fine foundry sand, which is used when a light color and fine texture are required. In medical centres one part Barium is added to two parts cement and five of sand where the walls need to block X-rays. When coating or rendering concrete surfaces a "splash" coat of one part cement to one of sand in liquid form is either thrown with a trowel or sprayed on the surface. This not only provides a better key for the render but prevents the porous concrete from sucking the water from it. For external work Portland cement is undoubtedly the best material on account of its strength, durability, and weather resisting External properties. If the plaster coat needs to be particularly strong and resistant to cracking, such as the walls of a squash court, Sizing is mixed with the plaster before application to increase both the surface bond strength and flexibility.

The first coat of render is from 1/2 to 3/4 inches thick, and is mixed, dependant on the surface to be covered, in the proportions of from one part of cement to two of sand to one part to six of sand. A ½ shovel of Lime is often added to make the mix more pliable. After dampening the surface to be coated, two horizontal bands of render called "screeds" are applied, one at around head height and the other just above floor level, these are then marked for vertical/horizontal alignment, finished, then allowed to partially dry. In a process similar to laying concrete, the wall is then rendered to a slightly higher level than the screeds, and using a "straight edge" (screed), the Plasterer uses the screeds as guides removing the excess render and leaving a rough flat surface. For a lower cost finish or if a rough surface is specified the screeds can be dispensed with. The render is then finished with a float (a smooth flat wooden tool with handle) to fill or remove larger imperfections. For some applications where a stronger key is required the surface is scored by later use of a float with nails protruding from the base. If the render is to be the finished surface then a float with a sponge attached to the base is then used on the wall until the surface is blemish free.

For quality work, or where the wall is out of plumb requiring a large variation in render thickness, a thin "scratch" render coat is first applied then a second coat finished as described above. After around 24 hours the render has dried but before the final plaster coat is applied a trowel is used to scrape loose sand grains from the surface which would otherwise spoil the plaster finish.

The finishing or setting plaster coat which is about 3/16nbsp;&inches thick is worked with a hand trowel on the surface of the rendering, which must first be well wetted. The plaster is applied in two coats to slow the drying speed of the second coat and after drying must still be wetted and worked for a time to produce a thin film of watery plaster which has the effect of "Polishing" the finished surface. This finishing plaster is normally trowelled on two or three passes to achieve this.