Cement is not often used for internal plastering, internal plasters are based on gypsum which is mixed with sand or used neat. The Thistle brand of plasters made by British Gypsum Ltd, include backing coats and finishing plasters.
All walls to be finished in gypsum plasters must be reasonably dry and protected from the weather. Raked out joints will improve the bond between the plaster and the background. Wooden linings for door openings, cupboard frames and other woodwork must stand proud of the brickwork by about 10 mm to allow for the plaster. The background must be brushed down with a hard broom to remove dust, loose mortar and salts.
Gypsum plaster does not need a long drying-out period so that the finish plaster can be applied to the undercoat fairly soon after undercoating has been completed. If the finished plaster is applied correctly it will be free from shrinkage cracks. Full strength is attained within a few hours of it setting and special treatment of angles is unnecessary.
Good adhesion is obtained by applying the undercoat with firm pressure. When building up a thick coating a thin layer should be applied first with firm pressure, then it is built up to the required thickness and levelled with a straightedge, against horizontal bands of plaster set up in the same way as those used when rendering surfaces with cement and sand mixes.
Thistle Browning is an undercoat which is suitable for most solid backgrounds and is applied as a floating coat of 11 mm thick. When brought to an even surface it is scratched to provide a key for the Thistle finish coating. This is applied about 2 mm thick and used neat. It is trowelled to a smooth surface using a steel plastering trowel.
There are a few problems encountered in plastering and those that occur are due mainly to the incorrect use of the materials. Cracks may be caused by structural movement and most commonly occur on ceilings where they are caused by the movement of the timber joists or shrinkage of concrete lintels. Other cracks which appear on the walls around lintels and window sills may be caused by settlement or by thermal movement. These cracks can be cut out and filled, but as they are likely to reappear the repair should be delayed as long as possible.
Crazing in the form of hair cracks in the finishing coat may be due to using a loamy sand in the undercoating or it could be that the finish has been applied over a cement or lime based undercoat before the undercoat has had time to dry out. The filling of so many fine cracks would be very difficult and the area may have to be lined with wallpaper.
Failure to let a cement or cement-lime based undercoat dry out sufficiently will also cause the loss of adhesion of the finishing coat. Other causes are the failure to key the undercoat properly and also adding too much sand to the undercoat, making it too weak. The only remedy for a poor backing is to strip it off completely and replaster using a stronger mix. Where only the finish coating is affected then it can be stripped off and the undercoat given time to dry thoroughly. Its surface should be roughened and all the dust removed with a damp brush. Then the surface can be replastered in the normal way.
Ceilings and walls can be lined with plasterboard which is either given a finishing coat of plaster or, if the board is fixed with the ivory side outward, it can be filled at the joints and decorated. If the boards are fixed with the grey side out then they need to be given a skim coat of Board Finish Plaster.
Ceiling boards are fixed first and then the walls are dry lined with the plasterboard. Ceiling joists must be placed so that they give adequate support to the boards, which for 1220 mm boards means 400 mm centres. The paper covered edges of the plasterboard need no extra support where they cross the joist but it is better if the cut edges of the boards are supported on noggings fixed between the joists where appropriate. The nails used must not cut through the paper covering and special galvanised or Sherardised nails are available. A solid fixing is needed around the perimeter of the ceiling to prevent the plaster pushing the boards up to make a wavy line when the walls are plastered. The end joints of the boards should be staggered, not in a straight line across the ceiling. Before plastering starts, the joints are filled flush with Thistle Board Finish. All internal and external angles and the angles between the walls and ceilings are reinforced with jute scrim not less than 90 mm wide. This joint filling should be left to set, but not dry out, before plastering commences.
The skim coat of finishing plaster is applied using a wooden float to spread the material as evenly as possible over the boards. The finish plaster is used neat and is mixed with water in a clean bucket. Add the plaster to the water stirring all the time, beating out any lumps and bringing the plaster to a creamy consistency.
When the plaster starts to set it is smoothed with a steel plastering trowel. A small amount of water applied with a brush just ahead of the trowel will help to ease the setting plaster to a smooth finish, but do not use too much water or over trowel the surface as this could make the finished work dusty.
Where it is necessary to plaster over glazed surfaces or walls finished with oil paints, old brickwork or other difficult surfaces, Carlite Bonding Coat or Welterweight should be used, in conjunction with a proprietary bonding agent. The surfaces must be wirebrushed to remove loose paint and other extraneous matter. Then they should be washed down with a detergent and rinsed, and be left to dry out thoroughly.
Carlite is a retarded hemihydrate pre-mixed gypsum plaster made by British Gypsum and requires only the addition of water to prepare it for use. The Bonding Coat is an undercoat for low suction backgrounds and the Carlite Browning is an undercoat for most solid backgrounds which offer an adequate mechanical key. It is applied about 11 mm thick and brought to an even surface with a straightedge then lightly scratched to form a key for the Carlite finish. The finish plaster is applied at 2 mm thick as soon as the floating coat has set. It is brought to a smooth surface using a steel trowel.
Dry lining walls
Instead of plastering the walls with wet material in the traditional way, they can be lined with plasterboards which are stuck to the brickwork with dabs of plaster or special adhesive. Normally the system is used to secure 900 mm wide, 9.5 mm or 12.7 mm h in thick, tapered-edge boards.
Installation starts by marking the walls vertically at 450 mm centres and then checking them with a straightedge to find the high spots. At these points bitumen-impregnated fibreboard pads are stuck in place with plaster. These pads are used as a guide for fixing the rest of the pads. Two pads are fixed at each angle, one 230 mm from the ceiling and the other 100 mm from the floor. They are carefully plumbed and then an intermediate pad is fixed half-way betweem them. This process is continued down all the marked lines, making sure that they are all in line horizontally and plumb vertically.
When the pads have set, which takes about two hours, Thistle Board Finish plaster or Gyproc multi-purpose adhesive is applied to the wall in a series of dabs between the pads, about three dabs between each pad vertically. Each dab should be the length of a plasterer’s trowel and thick enough to stand pround of the pads. There should be a space of 50 mm to 75 mm between the dabs and they should be between 50 mm and 75 mm wide. Only enough dabs should be applied for one board at a time and those at the joints should be set about 25 mm in from the line.
Next, taper edged boards are placed in position starting from an internal angle or a window reveal. The boards should be cut not less than 13 mm and not more than 25 mm shorter than the floor to ceiling height. The boards are pressed back against the pads using a straightedge. After ensuring that the leading edge is plumb and central on the joint pads, double-headed nails are driven through the boards and into the pads. They are generally only required at the edges.
The nails are driven until the first head slightly dimples the surface of the board which they will then hold in position for the critical setting period. These nails can be removed by pulling the second head with pincers. This process of fixing is repeated until the walls are fully lined. Joints should be only lightly butted and at internal angles if a board has to be cut, the cut edge is fitted into the angle.
Window reveals are lined with narrow widths of board cut so that they line up with the pads on the main wall face. These strips of plasterboard are fixed by applying dabs of plaster to their backs and pressing them into position. The lining to the window head will have to be temporarily supported. The main wall is lined from the window opening allowing the bound edge of the plasterboard to cover the cut edge of the reveal.
Dry lining is carried out using the ivory decorating side of the board outwards and the joints are filled with a special compound which is reinforced with a paper tape. The filler comes in 12.5 kg bags and is mixed by sprinkling it into water until a thick creamy consistency is achieved. When a layer of filler has been pressed into the joint the tape is applied and a second layer of filler is trowelled over the top. Before the filler starts to stiffen a sponge is damped and surplus material is wiped from the boards. When the filler has set, any depressions are filled in and smoothed over.
Next, joint finish is applied after the filler has set but not necessarily dried out. This material is applied in two layers allowing the first to dry before the second is applied. Each of the layers is feathered out to mask the joints completely.
Laying a floor screed
A fine topping or screed can be laid on the top of the base concrete of a floor in order to provide a finer surface than may be possible with the coarser concrete used for the structure. This topping, which can be from 25 mm to 50 mm thick is applied very much in the same manner as rendering. First, strips of topping mix are laid and in them lengths of wooden batten are bedded to make a firm and level guide from which to straighten up the filling between them using a straightedge. The battens are then removed and the hollows filled.
When the cement topping has started to set it is trowelled to a smooth surface using a steel trowel. The mix should be fairly dry having just sufficient water in it to make it possible to lay it. It should be smoothed carefully, trowelling the surface as little as possible because over trowelling will bring the cement to the top and cause the finished surface to be dusty.