Plaster and Skim Finishes

A good plaster finish starts with the background. Gypsum follows exactly what lies beneath it, so any flaw in the masonry will show through on the finished wall.

1. Check the background: it must be firm, dry and free of dust, grease and old paint residue. On concrete, remove any traces of release oil from the formwork.
2. Measure how flat the walls are, usually with a laser level, and work out how thick the plaster needs to be.
3. Cover windows, floors, sockets and switches. Fix corner beads and plaster stop beads where needed.
4. Prime the background. Smooth concrete gets a bonding agent so the plaster grips.
5. Apply the plaster by machine or by hand, continuously across the whole wall, without long breaks.
6. Rule the plaster off flat with an aluminium straightedge and fill any hollows.
7. Once the plaster starts to set, dampen the surface and trowel it smooth with a stainless steel trowel.
8. Let the plaster dry with gentle, steady ventilation.
9. Depending on the finish required, apply a thin skim coat, or two coats on more demanding walls.

A typical gypsum plaster coat is roughly 10 to 15 mm thick, while a skim coat is only about a millimetre. The exact figures are in the manufacturer's technical data sheet.

Machine-applied plaster is mixed and sprayed by a plastering machine. The mix stays the same consistency throughout and the work goes quickly, so it suits large areas in new builds and whole flats. It also reduces the risk of so-called cold joints, the visible lines where fresh plaster meets plaster that has already set.

Hand-applied plaster is mixed in smaller batches and laid on with a trowel. It suits repairs, small or narrow rooms, window reveals and places a machine cannot reach. It is slower, but the plasterer has better control over the details.

For skimming, two main groups of material are used:

• Gypsum finishing plasters, which naturally go well with a gypsum base coat
• Polymer finishing fillers, when a very smooth surface is needed, for example under paints with a sheen or where large windows throw light across the wall

Machine-applied gypsum plaster is often trowelled so smooth that a separate skim is not needed. If the wall will be lit from the side, or you want an almost glass-smooth surface, an extra skim coat is worth it. A fine lime finish is another option, but it gives a slightly textured, matt surface rather than a smooth one.

Gypsum plaster belongs in dry interior rooms: living rooms, bedrooms, children's rooms, hallways and offices. It can briefly take up moisture from the air and release it later, which makes rooms pleasant to live in.

Choose a different system if:

• The wall is external or the space is unheated, where gypsum is not used
• The room is permanently damp, such as a shower room, utility room or cellar
• The masonry suffers from rising damp or salts, typical in older houses without a damp-proof course
• The wall must withstand heavy knocks, for example in a garage or workshop

In these cases a lime-cement or renovation (salt-resistant) plaster is used, or plasterboard. In a domestic bathroom, manufacturers often allow gypsum plaster away from areas hit directly by water, but a shower enclosure needs waterproofing or a different plaster. Always check the technical data sheet for the specific product.

Simply replastering a home usually needs no permission. If you are also removing or moving partition walls or altering load-bearing structures, check with your local building authority first. In a block of flats, let the building management or owners' association know in advance, mainly because of noise, dust and carrying materials through shared areas.

The effort, and so the scope of the work, depends mainly on:

• The state of the background: new, straight masonry is simple, while old walls with loose plaster must first be scraped or hacked off
• Unevenness: the more crooked the wall, the thicker the coat and the more material
• The shape of the room: alcoves, sloping ceilings, reveals and lots of corners mean more handwork
• The number of skim coats and how smooth the finish must be
• Machine or hand application
• Access to the site: an upper floor without a lift, or no water or power on site

In older houses, plastering often comes with demolition work and wall straightening, so the full scope can only be judged on site.

In practice it often happens that a wall looks fine before painting and the faults only appear under the paint. The usual causes:

• Plastering onto damp or not yet cured masonry
• No primer, so areas with different suction dry unevenly and leave patches
• Stopping and starting, which leaves cold joints
• Working in the cold: manufacturers usually advise against working below about 5 °C
• Speeding up drying with heaters or draughts, which causes cracking
• Plastering while floor screeds are being laid and the air is full of moisture
• Skimming over plaster that is not yet dry

A rough rule often quoted is about a day of drying per millimetre of thickness, with good ventilation and at room temperature. Give the walls time before painting and always use a suitable primer or mist coat first.

Leave full plastering to a tradesperson, but you can skim a small wall by hand yourself. For ceilings and anything at height, it is better to call in a professional.

1. Remove loose material, dust and grease from the wall.
2. Fill larger holes and cracks with filler and let it dry.
3. Prime the wall as the skim manufacturer recommends.
4. Mix the finishing plaster with water exactly as instructed, only as much as you can use in time.
5. Apply a thin coat with a stainless steel trowel using diagonal strokes.
6. Once the surface firms up, trowel it smooth with another pass.
7. When dry, check the wall with side lighting and lightly sand any high spots.
8. Apply a second coat if needed and let it dry again.

When you take over the work, look at each wall in side lighting, for example with a lamp held close to the surface, and check:

• Flatness with a long straightedge, with no noticeable hollows or bulges
• Straight, crisp corners and edges around windows and doors
• A surface free of cracks, pinholes, scratches and trowel marks
• Even colour with no darker damp patches
• Clean junctions at ceilings, floors and door frames
• Sockets, windows and floors still covered and undamaged
• Advice from the plasterer on when you can paint and which primer to use