Indy Total Construction

Finishing an Imperial ceiling with a coat of plaster is carried out in a similar manner to finishing off a drywall partition. However, when working above your head (which often presents difficulties of its own) it is best to apply small amounts of plaster at a time to avoid tiring your arms.

Deal with the joints first, spreading a thin layer of plaster down the center of each one and pressing lengths of 2in wide nylon mesh or paper tape into the wet plaster with your trowel. Lightly trowel over the tape then apply another thin layer of plaster on top.

Divide into handy bays; fill in each bay with a thin layer of plaster, but not over the joints. Hold the trowel blade at 30° to the surface of the ceiling; the back edge about 1/Gin clear of the board to provide an even layer. Reduce the blade angle as the plaster spreads and pinch the back edge in as you complete the stroke to stop the plaster falling off. Work away from you to avoid flicking plaster into your face.

When you have filled in all the bays, go over the entire ceiling with another thin layer of plaster. Rule it off with a long metal straightedge to remove the high patches and show up the low spots, which should be filled with a thin coat of plaster.

How you treat the angle between the ceiling and walls depends on whether you are replastering the walls at the same time or not. If not, simply run the corner of the trowel blade along the angle from the ceiling and wall sides to cut out the angle neatly. If you are replastering the wall as well, lay on the floating coat then tape the joint between the wall and ceiling before applying the finish coats. Finish the corner as normal.

Finally, polish the hardened plaster with a clean, wetted trowel blade.

If you intend papering or painting directly over the drywall, the joints must first be made to “disappear”. For this you will need drywall joint compound, paper jointing tape and joint finish (see below).

First spread a layer of compound down the seam and, with a taping knife, press the tape into it. Apply another layer of compound over the top, feathering the edges by going over them with a damp sponge.

When the compound has dried, apply a finishing layer, feathering its edges in the same way. Treat the nail head depressions with compound and finish in the same manner.

At the angles between wall and ceiling, fill large gaps with compound; then apply compound to both wall and ceiling and press a creased length of tape into it. Apply two more layers of compound to wall and ceiling, feathering the edges of each one.

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.