Plaster grades and qualities for building work

Plaster

Plaster is a fine white powder, with the main characteristic that, after being mixed with water, it dries hard and unworkable very quickly, much faster than lime or cement and sometimes within a few minutes. Synthetic plaster is also available, but there is little reason why you would ever choose not to use natural / real plaster.

Unlike cement, plaster is used alone, mixed with water - no sand is used.

 

A couple of unexpected uses of plaster

One time when plaster is indispensable is when you are trying to hold tubing for electrical cables in place, before pointing a wall. A big dab of plaster every metre will hold the tubing solidly in place, and the rapid drying time means that within a few minutes you can let go.

Another time it is useful is when installing a bath. A big blob of plaster around the base of each of the spindly legs will help hold the bath in place much more solidly than the legs alone.

 

Mixing Plaster - a race against the clock

Measure the quantities carefully, according to the instructions on the bag. It is not possible to add more water later without weakening or spoiling the whole mixture. Make small quantities, because of the speed with which it dries.

To mix plaster, the powder must be sprinkled onto the water, otherwise dry lumps will be present in the final mix.

If a small amount of lime (CAEB - add 15g per litre of water) is added to the mix, the drying time will be slowed a little.

The plaster should then be left a minute or two before being vigorously stirred, ideally with a  drill with a mixer attachment, in a plastic or rubber tray or bucket.

After about 10 minutes the plaster should be a thick mix ready to apply. And after another 30 minutes will probably be unusably hard!

 

Other

Plaster can be combined with colourants to create interesting effects. This requires some skill and experience, since the plaster dries so fast.

Plaster will happily stick to cement, but curiously cement will not stick on to plaster - there is a chemical reaction produced which stops it sticking properly.

Because plaster is difficult to work with, most amateurs use it only for small repairs, using plasterboard for large areas of wall. It is also possible to buy tubs of ready mixed products which dry much more slowly and can be used, for example, for filling the gaps between sheets of plasterboard of blocking small holes.