The ins and outs of plastering
THE FIRST coat of your finished product, plastering hides the brickwork and hopefully gives a smooth surface which can then be tiled or painted. Like the mortar used in brickwork it is a mixture of sand and cement so it is important to ensure that the mix is correct. A mix with too much sand will be too soft and will break up easily. The finished product must feel hard when you scratch your fingernails over it.
As the plaster will form a “finished” surface it is important that the plasterer has the correct tools to do the job; check that his floats of either timber or steel are smooth and in good condition. The most important tool is his straight edge, check to see that it is in fact “straight” and that the edges are smooth and free of chips.
Ideally the finished product should be 12mm to 15mm thick – plaster thicker than this will tend to pull away from the wall. Any area that needs a thicker covering, for whatever reason, usually the brickwork being out of level, should be plastered in two coats.
Plaster that is too thin will tend to craze as it dries out and show the pattern of the brickwork behind. As an extra protection, waterproofing additives can be added to the exter- nal plaster mix. Your plaster is going to crack; these cracks occur naturally during the curing and shrinking process which happens after application. Exter nal plaster applied during strong winds will also show minor cracks, as the strong winds and heat tend to dry the plaster out too quickly. These minor cracks are inevitable and it is unfair to hold your builder liable and he cannot be bound to rectify them. To minimise the risks of cracking, keep the internal walls well ventilated, and it does not harm to slow down the external curing process by keeping the plaster damp for a few days once it has set.
Insist that your contractor finishes a complete area in one day, as joins in plaster will be visible. Walls should be plastered from corner to corner, and from floor to ceiling in one operation, using the same batch of mortar. Obviously reveals around window openings can be plastered later.
Talking of openings, check that all frames are protected and that they are washed off after plastering is complete for the day. As substrates such as brickwork and concrete have different properties, a “v” joint should be cut in the plaster where the different materials meet; this will ensure that when movement occurs the resulting crack will be in a straight line.
Plaster can be finished off in many different ways, but techniques such as stippling or Spanish plaster will leave a surface, which is more likely to collect dust and in my opinion should be avoided. For a really smooth internal finish, a finishing coat of cretestone can be used; this is really the ultimate finish and is often left unpainted, with just a clear sealer coat being applied. With the developments in paint technology an equally smooth finish can be obtained by using the appropriate paint, and the downside of unpainted cretestone is that it is very difficult to patch without the patch being visible.
Finally, if you are going to have moulded plaster features on the top of walls or around openings, insist on seeing samples first, and agree with the builder on the standard required. There are very few plasterers left who can really produce good quality mouldings, so check the quality on a daily basis and see that the plasterer is using a template.
Next year – CEILINGS
COVER STORY: It is vital that your plasterer has the correct tools for getting the right finish.