Plaster is bought in sacks as a fine white powder. It has the main characteristic that after being mixed with water it dries completely hard and unworkable very quickly. This happens much faster than lime or cement and sometimes within a few minutes (it well tell you on the sack an estimated time of use). 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, simply mixed with water (follow the instructions carefully) – no sand is used in the mixture.
A couple of unexpected uses of plaster
One occasion when plaster becomes indispensable is when you are trying to hold tubing (gaines) for electrical cables in place, before you do the pointing of a wall. A big dab of plaster every metre or so will hold the tubing solidly in place, and the rapid drying time of the plaster means that within a few minutes you can let go of the tube and it will be held in place.
Another time it is useful is when installing a bath. A big blob of plaster around the base of each of the 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 that it dries.
To make plaster, the plaster powder must be sprinkled onto the water, rather than adding water to the plaster. Otherwise dry lumps will be present in the final mix. A plastic or rubber tray or bucket is best for this.
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 for a minute or two before being vigorously stirred. Usually a drill with a mixer attachment is the most efficient way to mix.
After standing for a further 10 minutes the plaster should be a thick mix, ready to apply. And 30 minutes later it will probably be unusably hard!
Plaster can be combined with colourants to create interesting effects. This does require some skill and experience, since the plaster dries so fast.
Plaster will stick to cement, but cement will not stick on to plaster – there is a chemical reaction produced which stops it sticking properly.
Because (French) 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, or blocking small holes.