One problem the beginner faces when tackling a plastering job is that of producing a floating coat that is uniform in thickness and level over the entire wall. The answer is to divide the wall into sections and use the dividers as depth guides.
Space the dividers as close together or as far apart as you like, but a suitable distance is about 3ft.
There are various methods for dividing the wall into bays, and a traditional way is to trowel narrow strips of plaster from floor to ceiling. Known as “screeds”, these strips of plaster are allowed to harden, then more plaster is spread on the wall between them and brought up to their level, using a long straightedge placed across the screeds to check.
The problem with the screening method is being able to get the plaster strips to the right thickness in the first place. Small blocks of wood, known as “dots”, can be fixed to the wall at the top and bottom of the screed position and used as thickness guides by setting a straightedge between them.
An easier way is to use wooden “grounds”. These are lengths of planed, %in thick by about 2in wide softwood, which are fixed to the wall with masonry nails. Since you plaster only one bay at a time, you need only two grounds per wall and, therefore, you can move them along as you work.
After setting out the first bay, you can apply a floating coat between the two wooden grounds, striking it off level with a long wooden straightedge called a “rule”. Then, having let the plaster harden off for a while, you should carefully pull one ground from the wall and nail it back on further along the wall to make a second bay.
Continue applying the floating coat in this way until you have completed the job.
When fitting wooden grounds it is essential that they are set vertically, otherwise the plaster surface will be out of true. Use a long mason’s level to check that they are upright and, if necessary, slip small wooden shims as packing pieces behind the grounds to bring them into line.
An alternative to using wood grounds is the metal screed bead which you can buy from your builder’s supply house. It does the same job as the ground but is designed to be left in place on the wall; it disappears under the finishing coat of plaster.
The center of the bead is formed into a raised, inverted U-shape, the depth of which is equal to the depth of the floating coat, and on each side there is expanded metal mesh. You can cut it by snipping through the mesh with metal snips then sawing through the bead with a hacksaw.
Beading is fixed to the wall with “dabs” — blobs of plaster troweled on to the wall. Push the beading into the dabs then check with a level.
Allow the plaster to harden off and then use the beads as thickness guides for the floating coat.