How to Skim a Wall (Plastering Basics)

Skimming is the process of applying a thin finishing coat of plaster, typically 2–3 mm total, over a prepared surface to create a smooth, paint-ready finish. It is the last step before decorating on newly dry-lined walls, repaired plasterboard, or existing walls that have become too pitted and uneven to bring back with decorator’s filler alone. It is also the most intimidating DIY plastering task, because the working window is short and technique matters more than in most building trades.

Done badly, skimming leaves ridges, tears and trowel marks that are visible in raking light. Done well, it produces a surface almost indistinguishable from a professional finish. With the right materials and a methodical approach, a competent DIYer can skim an average-sized wall to a good standard, though it usually takes three or four attempts before the process feels natural.

Tools You Need

• 14-inch plastering trowel (stainless steel)
• Hawk (flat aluminium or plastic board for holding plaster)
• Mixing bucket (15–20 litre)
• Paddle mixer (attaches to a power drill)
• Smaller 6-inch skimming trowel or spatula for edges and corners
• Corner trowel (optional but helpful)
• Bucket of clean water and large paintbrush
• Scaffolding board or hop-up for ceiling-height work

Materials and Costs

A 25 kg bag of multi-finish covers about 10 m² at the standard 2 mm thickness, and proportionally less if you go thicker.³ For a standard 12 m² bedroom wall, budget on two bags.

Step 1: Prepare the Surface

Skimming over an unsuitable surface is the most common reason for failure. The surface must be:

• Stable, all loose plaster, efflorescence (salt deposits), and peeling paint removed
• Dust-free, brush and vacuum thoroughly
• Not too absorbent, bare breeze block, dry undercoats and other high-suction backgrounds will suck moisture from the skim coat before it can be worked flat

Plasterboard needs no primer: British Gypsum lists it as a low-suction background that Multi-Finish can go straight onto, applied to the face side of the board.³ For very absorbent backgrounds, British Gypsum specifies a proprietary primer (Thistle GypPrime); many plasterers use diluted PVA instead, applied so that it is tacky rather than wet when the plaster goes on.³ Stick fibre scrim tape over plasterboard joints and internal corners before skimming.

If there are significant hollows or lumps over 5 mm, apply a bonding coat plaster first and allow it to set (check the bag for timings) before the finish coat. Skimming cannot bridge voids, it will craze and crack if applied too thick in a single pass.

Step 2: Mix the Plaster

Pour clean water into the bucket first, about 11.5 litres for a full 25 kg bag (half that for half a bag).⁴ Slowly pour in the plaster while mixing with the paddle mixer. The target consistency is thick cream or Greek yoghurt: it should hold its shape on the hawk but flow when the trowel draws through it. Scrape the bucket sides and mix for a full 2 minutes. Do not add more plaster once mixing is complete.

Work in reasonable room temperature, plaster sets faster in warm conditions and on absorbent backgrounds, and more slowly in cold. British Gypsum gives Multi-Finish a working time of up to about 40 minutes.³

Step 3: Apply the First Coat

Load the hawk from the bucket and use the plastering trowel to sweep plaster from the hawk to the wall in long, upward strokes. Work a section at a time, typically 1.5–2 m wide, starting from the top and working down. Apply a first layer about 1 mm thick, spreading firmly to press the plaster into the surface.⁴ At this stage, do not worry about perfection: aim for even coverage at a consistent depth.

Press the top edge of the trowel slightly away from the wall (about 5°) on the upstroke and flatten it on the downstroke to reduce ridges. Move quickly, you need the first coat to be on before it starts to firm up.

Step 4: Apply the Second Coat

After roughly 20–45 minutes (the first coat should feel firm but not hard, test with a thumb), apply the second coat, bringing the total to at least 2 mm.⁴ This second pass fills any low spots and begins to consolidate the surface. Use longer, smoother sweeps, blending the edges into adjacent sections. At this stage the surface will still look relatively rough.

Step 5: Polish and Feather

This is where the finish is won or lost. As the plaster transitions from firm to hard (you will see the colour change from dark grey to lighter grey), wet the trowel face with the large paintbrush and work it across the surface in wide, flat arcs. The moisture at the trowel face lubricates the setting plaster and drags the cream to the surface, filling pinholes and trowel marks. Apply light, firm, consistent pressure. Work across the entire surface in overlapping passes.

On British Gypsum’s guide timeline, the plaster is ready to close in about 90 minutes after mixing, with trowelling passes at around 100 and 120 minutes.⁴ A second polishing pass 10–20 minutes later (using slightly less water) brings the surface to a near-mirror finish. Do not over-trowel: once the plaster starts to drag and tear, stop. It is setting and cannot be reworked.

Step 6: Cure and Dry

Leave the finished plaster undisturbed for at least a day; it sets within a few hours but takes much longer to dry. Fresh plaster shrinks as it loses moisture; keeping rooms warm and ventilated speeds drying but do not force it with a dehumidifier in the first 24 hours. Fully dry plaster is light pink-beige; damp plaster stays darker grey. Wait until the entire surface is uniformly pale before painting.

Apply a mist coat (emulsion thinned with water as the paint maker directs) to seal the fresh plaster before any topcoat. Skipping this causes the paint to absorb unevenly and pull away.

Common Mistakes and How to Avoid Them

Mixing too much at once. For a beginner, mix half a bag at a time, you will waste less plaster while you find your pace.

Applying too thick. A single heavy coat is harder to get flat and more prone to cracking. Two thin coats are always better.

Trowelling too early. Polishing wet plaster smears it rather than smoothing it. Wait for the colour to lighten slightly before final trowelling.

Skimming over damp walls. Plaster cannot adhere to a saturated substrate. Investigate and fix any damp source before plastering.

Not protecting floors and skirting. Plaster splashes set hard and can mark stone and timber. Sheet up the floor and tape skirting boards before starting.

If a wall has widespread repairs, a professional plasterer is genuinely better value than fighting a difficult substrate. A day rate of £150–£250 is well-spent for a whole room skim that would take a DIYer a weekend and produce a less predictable finish.