Building Plastering – Mixing and Surface Preparation

To mix cement plaster, the cement, with or without 10% of lime, should be mixed thoroughly with sand until the colour is uniform. The water is then added gradually while mixing continues and until the required consistency is obtained or until it is just plastic enough to be spread and to hold to the building wall. It must be used within 45 minutes of mixing and no plaster that has begun to set should be remixed with water and used.

To mix cement-lime plaster, the cement, lime and sand may be mixed dry in the required proportions, then mixed wet with sufficient water to give the required consistency. With this method, the plaster must be used within 45 minutes of mixing and no re-tempering of partly set plaster should be attempted. More often, the lime and sand are mixed together first, to form what is known as the ‘coarse stuff’, this allows for further hydration of the lime before using and makes it easier to apply, more plastic and quicker setting. With this method mix 1 part of lime with 6 or 41/2 parts of sand (depending upon whether the mix is to be 1:1:6 or 1:2:9), first dry and then with sufficient water to give a stiff mix, adding the water gradually. The mortar should then be formed into a heap, covered with wet sacks and left for a week to 10 days. Then, immediately before use, the coarse stuff should be mixed with cement, using 1 part cement to 6 or 9 parts of coarse stuff, adding water if necessary to bring to a workable consistency, just right for plastering a building. After the cement is added the mortar must be used up within 45 minutes and no re-tempering of partly set mortar should be attempted.

To mix lime plaster, mix 1 part of lime with 3 or 4 parts sand, first dry and then with sufficient water to make a stiff mix. Heap, cover over and keep damp and allow it to mature for 7 to 10 days. Then work up again, adding water if necessary and it is ready to use. Lime mortar, if kept damp may stand for 3 or 4 weeks before using. Sometimes, up to 10% of cement is added to the coarse stuff to quicken the setting, in which case the mortar must be used up within 45 minutes of mixing.

Another method applicable to either cement plaster or cement-lime plaster is to mix the lime with water to a thick cream and allow to stand for 24 hours to seven days before mixing with the sand.

The building surface to be plastered must be cleaned free from paint, oil, dust, dirt, etc. As it is difficult to make plaster adhere to a very smooth surface, it is advisable on brick and block building walls to rake out the joints about ½ in. to provide a mechanical key, or to strike off the joints roughly with the face of the building wall relying on the roughness to form a key. Joints should be raked while the mortar is still soft.

Dense concrete blocks should have a rough surface, which is best obtained by wire brushing while they are still ‘green’ and the moulds should be treated with whitewash instead of oil. Dense in-situ concrete should also be wire brushed while still ‘green’, since, if allowed to harden, the surface will probably be so smooth that it will have to be hacked to provide a satisfactory bond for the building plaster.

If a key is not formed by roughening or hacking the surface of dense concrete, either cast-in-situ or in blocks, a ‘spatter-dash’ coat of mortar should be applied. This consists of a mix 1 part cement to 1 ½ parts of coarse sand first mixed dry and then with sufficient water to give a fairly wet mix. This is dashed onto the building wall in an uneven manner, usually with a coarse brush, using a strong whipping motion at right angles to the face of the building wall. This coat should be kept damp for at least two days and then allowed to dry out.

New brick and concrete walls should be allowed a reasonable time to dry out before plastering. When the wall has been prepared by cleaning, raking joints, roughening or given a ‘splatter-dash’ coat, it is ready for plastering.

If the building wall is irregular and out of plumb and it is desired to have the finish plaster surface a true plane, it is necessary to ‘make up’ or ‘dub out’ the low spots by successive making-up coats, each about 3/8 in. thick. These should not be carried to a ‘feather-edge’ but should stop back forming a ledge about 1/8 in. thick. When the making-up coats have brought the building wall to a reasonable plane surface the actual plaster coats can be applied.