WORKING WITH PLASTER

A plaster veneer wall being troweled smooth.

A plaster veneer wall being troweled smooth. (Photo credit: Wikipedia)

Patching small areas of plasterwork is a fairly straightforward job, but sometimes you’ll need to replaster a whole wall.

Before you can start, you have to learn the basic techniques.

Plaster is used on internal walls to give a smooth, flat surface that you can decorate with paint, wallpaper or tiles. There are two basic types of plaster in common use. One is a mix based on a mineral called gypsum. The other, cement-based plaster, is used mostly as ‘rendering’ to weatherproof the exterior walls of a house. But it is also employed indoors, especially as part of the treatment of damp walls, or as an ‘under-coat’ for other plasters. Its disadvantages are a slow drying time and the possibility that mistakes in proportioning of the con-stituents could result in a weak mix.

Types of plaster

Gypsum-based plasters have largely superseded cement-based plasters. They are quicker-setting and usually available in pre-mixed form, which requires only the addition of clean water to make them workable. Another point in their favour is that they contain lightweight aggregates such as perlite and vermiculite instead of sand, so they’re easier to use.

Ready-mixed plaster is usually spread onto the wall in two parts. The first is a backing or ‘floating’ coat, which is applied fairly thickly – up to 10mm (%in) – to take up any unevenness in the wall. The second is a finishing coat, which is spread on thinly – up to 3mm (Vsin) – and finished to give a smooth, matt surface.

Carlite is the most widely-used lightweight pre-mixed gypsum plaster and it’s available in various grades for use on different wall surfaces, depending upon how absorbent they are: the plaster will crack if the wall to which it’s applied draws moisture from it too quickly. Common brickwork and most types of lightweight building blocks, for instance, are described as having ‘high suction’, which means that their absorption rate is rapid. Concrete, engineering bricks, dense building blocks and plasterboard, on the other hand, have ‘low suction’.

You can recognise which walls are high-or low-suction by splashing on a little clean water. If it’s absorbed immediately, the wall is high-suction, but if it runs off the surface the wall is low-suction. If after this test you’re still unsure, you can treat the wall with a coat of PVA bonding agent or adhesive, which, when brushed on, turns all backgrounds into low-suction, and both seals and stabilizes the surface.

For a high-suction background you’ll need Carlite Browning plaster for the base coat; for low suction choose Carlite Bonding plaster. Use Bonding where the wall is of a composite nature (containing both high- and low-suction materials). Carlite Finish plaster is used as the final coat on Bonding and Browning plaster.

Preparing the surface

You’ll achieve a smooth, flat and long-lasting plastered finish only if you’ve prepared the background properly. If you’re replastering an old wall of bricks or blocks, hack off all the old plaster using a club hammer and bolster chisel and examine the mortar joints. If they’re soft and crumbly, rake them out and repoint them. Lightly dampen the wall using an old paintbrush – this is essential if the new finish is to stick properly, and prevents the wall absorbing too much moisture from the plaster. New brick or block walls probably won’t need any preparation before you plaster other than light wetting.

Smooth surfaces such as concrete and timber (used as lintels over doors and windows, for example), must be keyed to accept the plaster. You can do this either by nailing expanded metal laths – a 300mm (1 ft) wood, aluminium or plastic square with a handle, used to carry plaster to your working area.

– plasterer’s trowel (B) – the basic tool for applying plaster to the wall; it has a thin rectangular steel blade measuring about 250 x 115mm (10 x 41/2in) and a shaped wooden handle.

– wood float (C) – generally used to give a flatter finish coat to plaster, it is rectangular in shape. It can be converted to a devilling float for keying surfaces by driving in two or three nails at one end so that their points just protrude.

– rule – a planed softwood batten measuring about 75 x 25mm (3 x 1 in) and about 1.5m (5ft) long, used to level off the floating coat when applied between screeds, grounds or beads.

– water brush – used to dampen the wall and to sprinkle water on the trowel when finishing

– spirit level – for positioning the timber grounds accurately.

MAKING A SPOT BOARD

The spot board is used to hold the mixed plaster near the work area. Make one from aim (3ft) sq panel of exterior grade plywood mounted on an old table or tea chest so it’s at a convenient height. Make sure it projects over the edge of the stand so you can place the hawk underneath when loading with plaster.

BUYING PLASTER

Large quantities of plaster are sold in 50kg (110lb) bags: smaller amounts for patching are sold in 2.5 to 10kg (51/2 to 22lb) bags.

HOW FAR WILL IT GO?.

– 10kg (22lb) of Carlite Browning laid 10mm (3/ain) thick will cover about 1.5sq m (1.8sqyd).

•10kg (22lb) of Carlite Bonding laid 10mm (3/sin) thick will cover about 1.6sq m (1.9sqyd).

– 10kg (22lb) of Carlite Finish will cover about 5sq m (6sq yd).

Applying the plaster

Plaster is applied to the wall in a series of sections called ‘bays’. These need to be marked out. One method is to use timber battens called ‘grounds’ lightly nailed vertically to the wall. Another method employs ‘screeds’, which are narrow strips of plaster. These are spread onto the wall from floor to ceiling, generally using wood blocks called ‘dots’ at the top and bottom as thickness guides.

The distance between these markers can vary according to your skill in applying the plaster, but 1m (3ft) is an easily manageable width for the beginner. Screeds and grounds are essentially guides that enable you to apply the backing coat to the correct thickness, and when the plaster’s been applied to one bay it’s smoothed off level with them using a timber straight edge called a ‘rule’.

Expanded metal screed beads for flat surfaces and angle beads for external corners and refix it.

Plaster this second bay using the edge of the first one as a thickness guide, and rule off the surface carefully. Carry on in this way until you’ve covered the whole wall.

To ensure the finishing plaster will adhere to the backing coat the latter must be ‘keyed’ using a tool called a devilling float. This is a wooden or plastic block with nails driven in from the top so that their points just protrude through the base, and it’s used to scratch the surface of the backing coat lightly.

Two thin coats of finishing plaster will give a smooth and flat surface. The first coat is applied from bottom to top, working left to right if you’re right-handed, right to left other- wise, and is then ruled off. The second coat is applied straight away and then flattened off to produce a matt finish. When this has been done you return to the starting point and, with the addition of a little water splashed onto the wall, you trowel over the entire surface. When the plaster has hardened, trowel the surface again several times, applying water to the surface as a lubricant to create a smooth, flat finish.

Mixing the plaster

Cleanliness in mixing plaster is of prime importance because any dirt or debris that gets into the mix could affect the setting time and mar the finish. Keep a bucket of water nearby for cleaning the tools and don’t use this water for mixing the plaster – use clean, fresh tap water.

MIXING PLASTER

When mixing plaster you’ll need the following equipment:

– a trough about the size of a galvanized bath for large amounts, or a tea chest lined with polythene

– a 5 gallon (22 litre) bucket for small amounts

– a clean bucket containing clean water for adding to the mix

– a bucket to transfer mixed plaster to the spot board

– a clean shovel for stirring large mixes.

TIP: FIXING GROUNDS

It’s important that the timber grounds are fixed truly vertical as they’re guides to the thickness of the floating coat. On uneven walls, pad out gaps between the grounds and the wall with wood offcuts to make their faces vertical.

USING METAL BEADS

Expanded metal lath and plaster beads act as a thickness guide for applying plaster to the wall and levelling it off. They’re bedded on plaster dabs and remain in the wall when plastering is complete.

• Screed beads are used instead of timber grounds or plaster screeds.

• Angle beads fit over external corners and protect the plaster from chipping.

• Stop beads are used for plastering up to doorways or abutting skirtings

L.

Basic techniques: Floating

Patching small areas of damaged plaster-work is fairly straightforward but plastering a whole wall calls for a degree of skill in using the various tools that can only be achieved by practice.

When you’ve mixed the plaster place it on the spot board. If you’re right-handed, hold the hawk in your left hand and the trowel in your right (vice versa if left-handed). Grip the trowel so that your index finger is against the front shank, the toe of the trowel pointing left. The knuckles of your right hand should be uppermost. The hawk should rest in the left hand on your thumb and index finger.

To load the hawk, place it under the edge of the spot board, scoop a small amount of plaster onto the hawk and move it away.

Hold the hawk level and place the bottom edge of the trowel on it, blade at right-angles to the hawk face. As you push the trowel forward against the plaster, tilt the hawk until it’s almost vertical, keeping the trowel at right angles to the hawk face. Push the plaster off the hawk and gently slide it all back. Don’t drop it from too great a height or too fast as it will splash. Repeat this several times before attempting to spread the plaster onto the wall.

When you’re fairly confident, move to the wall and repeat the operation, but only remove half the plaster from the hawk. Keeping the trowel horizontal, place the lower edge hard against the wall at chest height. Open the gap between trowel and wall to about 5mm (V-dn). tilt the trowel up until its face is at about 30° to the wall surface and then move the trowel upwards. The gap is similar to a valve and controls the thickness of plaster applied to the wall. As the material is spread evenly and disappears from under the trowel, decrease the angle between trowel and wall so that you apply the last of the plaster with a pinching movement between the trowel edge and the wall. This prevents the plaster from sliding down. Repeat this until you get the plaster to stay on the wall.

After your ‘practice run’, scrape the plaster from the wall, and apply a ‘floating’ coat of backing plaster between the grounds; don’t worry about any ridges or hollows at this stage but aim to get coverage of an even thickness all over.

Rule over the plaster and fill in any hollow areas, then rule again. Before the plaster has set, lightly key it with a devilling float.

Basic techniques: Finishing

Carlite plaster sets in less than two hours, so you should apply the finish coat as soon as possible after the floating coat has hardened. Use the hawk and trowel as if applying the floating coat, but take less plaster onto the hawk and apply a very thin coat to the floating coat, working left to right and from bottom to top. Smooth out all ridges to leave the surface as flat as possible. Once you’ve covered the undercoat, repeat the operation. Lightly sprinkle water onto the face of the trowel using a brush. With the trowel blade at an angle of 25 to 30°, trowel over the finish coat with long straight sweeps to achieve a smooth, flat finish.

Leave the plaster until set. Then trowel once more, aided by water and harder pressure, to polish the surface.

Sorry, comments are closed for this post.