A skim coat (also called a finishing coat or skim plaster) is a thin 2-3mm layer of fine gypsum plaster applied to the face of plasterboard or over a browning undercoat to produce a smooth, flat, paintable surface. It is the final stage of internal plastering. Skim sets quickly (typically within 1-2 hours) and must be applied confidently and quickly - spread thinly across the surface, then worked twice or three times with a steel trowel as it stiffens, pulling it down to a flat, near-perfect surface.

What is the difference between skim coat and backing plaster?

Backing plaster (also called browning plaster or undercoat plaster) is a coarser, thicker plaster applied in one or two coats at 8-11mm total depth to provide a flat, keyed surface over masonry walls. Skim coat is the thin, fine finishing plaster applied at 2-3mm over the hardened backing coat (or directly over plasterboard) to produce the final smooth surface. Backing plaster on its own is too rough and porous to be painted directly - skim coat is essential to produce the smooth 'plaster' finish that most internal walls have.

How long does skim coat plaster take to dry?

Skim coat sets chemically within 1-2 hours - it will be firm to the touch but not fully dry. Full drying (evaporation of excess water) takes 3-5 days for a fresh skim over plasterboard in normal conditions, or longer in cold, damp weather. A correctly dried skim will be uniformly pale - dark patches indicate damp areas that have not yet dried. Applying paint or wall coverings over incompletely dried skim causes blistering, bubbling, and adhesion failure. Mist coating (diluted emulsion) is recommended as a first coat to allow the skim to continue drying without being sealed.

When should I use skim coat vs. taped drywall finish?

Skim coat (full plastering) produces a harder, more durable surface that resists impact and is standard in UK domestic construction. Taped drywall finish (using joint compound and tape over board joints, with no full skim) is quicker and standard in North America but is softer and less durable - it also requires more skill at the joints to achieve invisibility. In the UK, full skim is expected in domestic properties; taped drywall finish is used in some commercial fitouts and partition systems where speed matters more than durability.

Finishes noun

Skim Coat

/ skɪm kəʊt /

Also known as: finishing plaster, skim plaster, finish coat, thin coat plaster

Definition

A skim coat is a thin 2-3mm layer of fine gypsum finishing plaster applied over plasterboard or a browning undercoat to produce a smooth, flat, and hard surface ready for decoration. It is the final stage of internal plastering. Skim sets rapidly (typically within 60-90 minutes) and must be applied quickly in two passes with a steel trowel - the first to lay the material on, the second to pull it flat and tighten it to a smooth finish as it begins to stiffen.

In practice

Skim plaster is typically mixed to a thick, lump-free cream consistency - too thick and it drags, too thin and it runs and will not hold. The surface to be skimmed must be primed (PVA or plasterboard primer for new board; mist coat of diluted PVA for absorbent backgrounds) to control suction. Mixed skim is applied with a hawk and a large steel plastering trowel, spread to 2-3mm in a single sweep, and worked with overlapping strokes to even out the thickness. A second, smaller quantity is applied over the first to cover any thinner areas.

As the plaster begins to stiffen (typically 20-40 minutes after application, depending on room temperature and surface suction), the plasterer works back across the surface with a clean, damp trowel, using arching strokes to flatten and tighten the surface. The goal is a near-zero texture with no trowel marks or lines. A slightly wet sponge float can be used to moisten the face if it is drying too quickly. Once fully set but not yet dried, a final light troweling with a barely damp trowel closes any remaining pinholes. Full drying before painting takes 3-5 days under normal conditions.

Building Regulations

StandardBS EN 13279 - Gypsum binders and gypsum plasters

Flatness toleranceSR2: +/-3mm under 2m straightedge for standard finish

Over fire-rated boardDoes not reduce fire resistance of the board below

Skim coat plaster has no direct Building Regulations requirements of its own, but forms part of systems that must meet Part B (fire), Part E (sound), and Part L (thermal) requirements. Applying skim coat over correctly specified and fixed plasterboard does not compromise the fire resistance or sound insulation performance of the system. Surface regularity standards for plastered surfaces are defined in BS 8481 and the NHBC Standards for new housing.

Full Building Regulations guidance

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Skim Coat

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