Wall Skimming

The process is much the same for walls and ceilings. Quality depends mostly on patience between the steps.

Preparing the surface

The surface must be clean, sound and dry. Dust, grease, flaking old paint and loose plaster come off first. Larger holes and cracks are filled before skimming. Then a primer or bonding agent goes on, as recommended by the skim manufacturer. It evens out how much water the wall absorbs and helps the skim stick.

Applying thin coats

The skim is spread with a stainless steel trowel, usually in two coats. The first evens out the surface and covers defects, the second creates the final smooth finish. Each coat is usually about 1 to 3 mm thick; the exact figure is on the bag or tub. Depending on the product, the second coat goes on once the first has firmed up or fully dried, so follow the manufacturer's instructions.

Sanding and final priming

Filler-type skims are sanded once dry with a fine mesh or paper, typically around 120 to 180 grit. This removes trowel marks and small ridges. Traditional gypsum finishing plaster, common in the UK, is usually polished with the trowel as it sets and needs little or no sanding. Either way, the surface is then dusted off and primed or mist-coated so the paint dries evenly without patches.

On a wall you see your work right in front of you and stand upright. Ceilings are harder for several reasons:

• Working overhead is tiring and it is harder to keep even pressure on the trowel
• A thick coat can sag or drop under its own weight, so thinner coats are used
• Sanding dust falls straight onto you, so dust extraction and a mask are essential
• Ceilings are often seen in low-angle light from the window, which shows every ripple

On a plasterboard ceiling the joints come first: tape goes into the joint compound, gets covered with another layer and is sanded once dry. Screw heads are filled too. Only then does the full skim go on. On concrete ceilings, the usual tasks are filling formwork marks, pores and the steps between precast panels.

Gypsum-based skim

The usual choice for dry living spaces such as living rooms, bedrooms, hallways and kitchens. It spreads easily, dries fairly quickly and finishes smooth. It is not suitable for damp areas such as bathrooms or cellars.

Cement-based skim

Harder and more tolerant of moisture, so it is used in bathrooms, utility rooms and plant rooms. It takes longer to dry and is harder to sand, so it needs to be applied accurately from the start.

Ready-mixed skims

These come as a paste in a tub and need no mixing. They suit small repairs, details and a fine final coat. For levelling large, uneven areas they are less suitable.

How much work is involved, and therefore the final cost, depends mainly on the state of the surface. A flat, sound wall often needs just one skim and a light sand. The work grows with:

• Uneven, cracked or greasy walls that need more coats and more preparation
• Several layers of old paint that have to come off first
• Lots of corners, window reveals, alcoves and cable chases
• High ceilings, stairwells and galleries that need scaffolding
• Cement-based skim, which is slower to work and harder to sand
• A furnished room that has to be cleared and well protected from dust

• Skipping the primer. An absorbent surface draws water out of the skim, which then bonds poorly, cracks or flakes
• Coats that are too thick. They crack as they dry and may drop from a ceiling. Several thin coats work better
• Not enough drying time. A new coat on a damp one can ripple or come away
• Poor conditions. Draughts, cold or high humidity cause uneven drying and fine cracks
• Sanding without checking in the light. Flaws often appear only after painting. A work lamp held close to the wall at a low angle shows them early
• Working without protection. Fine gypsum dust irritates the lungs and eyes, so wear a dust mask and goggles, and ideally use a sander with extraction

You can manage a small wall in a living space yourself if you have patience. Ceilings, high rooms and large areas are better left to an experienced decorator or plasterer.

1. Clear the room and cover the floor and furniture with dust sheets. Switch off the circuit at the consumer unit before loosening socket and switch faceplates.
2. Remove dust, grease and loose plaster.
3. Fill larger holes and cracks and let them dry.
4. Apply primer following the manufacturer's instructions.
5. Mix the gypsum skim as described on the bag, only as much as you can use before it starts to set.
6. Apply a thin first coat with the trowel and level the worst unevenness.
7. Let it dry or firm up as the manufacturer advises, then apply the second, final coat.
8. Once dry, sand with fine mesh wearing a mask and goggles.
9. Check the surface with a lamp held at a low angle and fill any small dents.
10. Vacuum off the dust and apply the final primer or mist coat.

If you live in a block of flats, check the house rules or ask the building management or owners' association when noisy and dusty work is allowed.

Before painting, look at the surface in low-angle light and run your hand over it. When accepting the work, check that:

• The surface is smooth, with no trowel marks or sanding scratches
• Plasterboard joints and screw heads are not visible
• No ripples or dents show in side light
• The skim is not cracking and does not sound hollow when tapped
• Corners and edges are straight and clean
• The surface is free of dust and has been primed
• The room is tidy and surrounding surfaces are clean