Repairing cracked plaster sometimes turns into a much bigger job than expected: you find a whole wall that needs replacing. Plastering a complete wall or ceiling is a fairly ambitious job and some skill is required to produce a true, flat surface. If you doubt your ability, it would be a good idea to experiment first on a suitable ‘hidden’ surface like a garage wall or ceiling or get a Plastering Company to do it for you.
Plastering a wall
It is unlikely that your wall will be perfectly flat and upright, so the first job is to find the high spots and determine whether or not the wall is out of true.
To do this, hold a spirit level on the back of a long straight-edged rule and move it systematically over the wall. Once you have prepared your surface, taking particular care to ensure that it has a good key, mix your mortar floating coat and start to lay ‘screeds’ on the wall. These are strips of plaster about 200mm wide which act as depth guides. They also break up the wall surface into easily manageable sections. With the steel laying trowel, lay the first screed a little over 13mm deep, from the floor to the ceiling on one side of the wall. Take it right up to the end of the wall. If this is a reveal corner, and you are using a length of angle bead to make a neat edge, place the bead in position before you lay the screed. If you are using the more traditional method, leave the reveal until you have floated the whole wall.
When you have laid the first strip, use a straight-edged rule about 1.8m long to rule it off. Hold the rule vertically and move it gently up and down from the outer edge of the screed inwards. Test for plumb with the straight edge and spirit level, and adjust, if necessary, with a little more careful ruling. Add more mortar if required. The ruling should reduce the thickness to about 13mm if the surface is good. A screed similar to the first one should then be laid on the other end of the wall. You should lay a minimum of two vertical screeds although, for ruling off later, it will help you to lay additional screeds at about 1.5m intervals. Now lay a horizontal screed about 50mm from the floor to join the vertical ones. Use the latter as guides when ruling off this screed. Another horizontal band should now be laid about 1.5m from the floor and ruled in the same way. The final screed should be laid across the top of the wall, flush with the ceiling, and ruled.
The screeds should now be smoothed with the plastic float. The sections between the screeds are then to be filled in flush. Deal with one section at a time and apply the floating mortar with the laying trowel. Rule off each section using the screeds as guides. Fill and re-rule if necessary. The 50mm strip at the bottom of the wall can be left if skirting board is to be used). When the whole wall is covered, clean the internal angles with the laying trowel, and wash down any adjacent surfaces smeared with mortar. Finally, go over the entire surface with the devil float to flatten any small bumps and to key the surface.
If you are using a sanded floating mix, allow it to dry for 24 hours; a lightweight plaster will take about four hours. If you are using the traditional method, any reveals in the wall should now be plastered with both the floating and the setting coat. Before applying the finish plaster, it is wise to test the surface for suction. Brush water on to a small section of the wall and watch what happens. If it is ‘sucked’ straight through, there is excessive suction. As this can have a disastrous effect by weakening the final coat, it will have to be remedied. Throughly dampening the wall will be sufficient in many cases, but if the suction persists, brush on a weak mixture of water and pva bonder-one part bonder to six parts water is about right-arid follow up immediately with the setting coat. When you are ready to apply the setting coat, mix up some finish plaster in a bucket and pour it on the spot board. Then clean out the bucket and fill it with clean water so that it is ready for another mix. Start to skim on the plaster with the laying trowel. First skim a band, about a trowel width, along the top. Then skim over the whole wall in vertical strips, up to the band already laid.
When the wall is covered, use the wood float to put on another thin application, again with vertical strokes. Work systematically from the left to the right (if you are right handed) and keep the seams well pressed down. Now, still using the wood float, Put on another thin application, but this time use horizontal strokes. At this point use the feather-edged rule to rule out the internal angles. Any seams still visible should be smoothed over with the wood float. If you are using an anhydrous plaster such as Sirapite, a little water will help for this, but do not use any at this stage if it is a hemihydrate. The final application of finish plaster should now be put on the wall. Use the steel trowel and lay a ‘tight’, or firm, coat with long vertical strokes. Then wash the trowel and sprinkle a little water over the plaster. Quickly follow up with the trowel and smooth over the whole area. Use the angle trowel to finish the internal angles and, finally, scrape any unwanted plaster off the floors, adjoining walls and the ceiling before it sets.