Summary

Skimming — applying a finish plaster coat over plasterboard or existing backing — is the standard way to achieve a smooth, paint-ready wall surface in UK residential construction. It is distinct from full plastering (applying a scratch coat and float coat over brick or block), though the finishing technique is similar.

A skim coat is typically 3-5mm thick and applied over plasterboard (already jointed and taped), Thistle bonding plaster, or an existing rough plaster surface that has been prepared with a bonding agent. The work demands proper mixing technique, the right consistency, fast working before the plaster sets, and methodical trowelling to a finish. Most plasterers complete a standard bedroom in 2-3 hours; a living room 3-4 hours.

This guide covers the tools, materials, mixing ratios, two-coat method, and the most common mistakes that lead to a poor finish.

Key Facts

  • British Gypsum Thistle MultiFinish — the most widely used finish plaster in the UK; sets in approximately 60-90 minutes
  • Thistle Board Finish — specifically formulated for plasterboard; finer gypsum particles; marginally better adhesion to board
  • Mixing ratio — approximately 1kg of powder to 0.7L of water; mix to a thick creamy consistency; not too stiff, not too runny
  • Pot life — once mixed, finish plaster is workable for approximately 45-60 minutes at 20°C; less in hot or dry conditions
  • Two coat method — first coat: 2-3mm; second coat applied before first goes off: 2mm; total 3-5mm
  • Trowel types — 11" or 14" stainless steel finishing trowel; large 16-18" trowel for experienced plasterers on large walls
  • Hawks — hold plaster while applying; 300mm or 350mm aluminium hawk
  • Feather edge — 2.4m or 1.8m aluminium straight-edge; used to check flatness and cut off high spots
  • Bonding agent / PVA — must be applied to porous substrates before skimming; diluted 5:1 (water:PVA) or use proprietary bonding agent; allow to go tacky (not wet) before plastering
  • Plasterboard joint tape — must be embedded in base coat; paper tape is standard; fibreglass mesh tape is also used but is more prone to cracking under skim
  • Setting time in cold weather — plaster sets much more slowly below 10°C; conversely accelerates above 25°C; never plaster in frost
  • Drying time before painting — 2-4 weeks for a full 5mm skim; the surface will lighten from dark pink to pale pink/cream when dry
  • BS EN 13279 — gypsum binders and plasters; the UK standard for finish plaster products

Quick Reference Table

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Stage Product Thickness Timing
Boarding primer / PVA Thistle Board Seal / diluted PVA 5:1 N/A Apply, let go tacky, then skim
First coat skim Thistle MultiFinish or Board Finish 2-3mm Apply flat; scratch lightly
Second coat skim Same batch as first coat 2mm Apply before first coat goes off
Trowelling out — — Trowel in stages as plaster firms
Final close — — Dampen with clean water brush; final trowel pass
Dry time — — 2-4 weeks before emulsion

Detailed Guidance

Tools and Equipment

Essential tools:

  • Mixing bucket (10-litre at minimum; 20-litre for large rooms)
  • Power drill with mixing paddle
  • Aluminium hawk (300 or 350mm)
  • 11" or 14" stainless steel finishing trowel
  • 1.8-2.4m aluminium feather-edge rule
  • Corner trowel (internal and external) or flexible angle bead
  • Brush for dampening and cleaning
  • Spot board (large piece of plasterboard or hardboard to hold mixed plaster)

Optional but useful:

  • Angle grinder or multi-tool for cutting beads
  • Spray bottle for damping surface during trowelling
  • Sponge for cleaning trowel mid-work

Preparation — Critical Step

Plasterboard must be correctly prepared before skimming:

  1. Ensure boards are well fixed — no movement, all screws driven flush (not below surface); gaps over 3mm between boards must be filled first
  2. Joint tape and base coat — scrim tape joints or apply joint tape with a bedding compound; allow to set before skimming
  3. Apply angle beads — external corners need metal or plastic angle bead; fix with dabs of bonding or screws; check plumb
  4. PVA / bonding primer — brush diluted PVA (5 parts water, 1 part PVA) over the entire surface; allow to become tacky but not completely dry (approx. 20-30 minutes)

On previously plastered surfaces being rescimmed:

  1. Check adhesion of existing plaster — tap to find hollow areas; hack off any loose areas
  2. Dampen the surface with water
  3. Apply Thistle Bonding or Hardwall base coat to build up level
  4. Allow base coat to set; score lightly; then skim

Mixing

  • Always add powder to water — not water to powder
  • Mix to a smooth, creamy consistency; no lumps; approximately the consistency of thick yoghurt
  • Mix in batches sized to your working speed — a first-time skimmer should mix half-bucket at a time; an experienced plasterer a full bucket
  • Do not retemper (add more water to a setting batch); this severely weakens the plaster
  • Clean mixing paddle and bucket before each new batch

Two-Coat Application Method

First coat:

  1. Scoop plaster onto the hawk with the trowel
  2. Apply to the wall in upward sweeps, covering an area of approximately 1m² at a time
  3. Work quickly to cover the wall before the first area begins to set
  4. Once covered, use the feather edge to rule off the surface — hold at each end, drag across the wall in long overlapping passes, removing high spots
  5. The finished first coat should be flat but not yet trowelled smooth

Second coat:

  1. Apply the second coat over the first coat while the first is still green (wet but firming)
  2. Lay on the second coat slightly thinner than the first (1.5-2mm)
  3. Rule off again with the feather edge
  4. Begin trowelling — keep the trowel at a low angle (15-20°) to avoid drag marks
  5. Work systematically from top to bottom

Trowelling sequence:

  1. First trowelling pass — flatten high spots and fill hollows while still wet
  2. Allow to firm slightly (5-10 minutes)
  3. Second trowelling pass — firm up the surface; more pressure; trowel kept clean
  4. As the plaster begins to go off, dampen lightly with a clean water brush and final-trowel for a polished finish

Common Faults and How to Avoid Them

Fault Cause Fix
Plaster dropping/slipping off wall PVA not tacky enough; plaster too wet Wait longer after PVA; reduce water ratio
Chatter marks (ridges from trowel) Trowel at wrong angle; plaster too firm Keep trowel at 15° angle; work faster
Hollows showing First coat too thin; poor suction control Build first coat more; apply to firmer board
Cracking during drying Applied too thick; plaster contaminated; drafted out Keep below 5mm; no direct heat; exclude draughts
Surface goes chalky Mixed too dry; or insufficient trowelling Improve mixing consistency; trowel before fully set
Joints showing through Scrim not embedded; different suction rates Tape joints properly; allow to set before skimming

Drying and Painting

Never paint fresh plaster with standard emulsion while it is still wet (dark-coloured). The surface must be pale pink/cream and feel dry to touch.

  • Minimum 2 weeks at normal temperature (15-20°C) before mist coat
  • 3-4 weeks in cold or poorly ventilated conditions
  • Never force-dry with heaters — cracking and surface defects result
  • First coat must be a mist coat (emulsion diluted 10% with water) — see interior emulsion

Frequently Asked Questions

Can I skim over existing emulsioned walls?

Yes, but preparation is critical. Wash the surface thoroughly with sugar soap, rinse, and allow to dry. Sand any glossy areas. Apply a bonding agent (e.g. Thistle BondIt or diluted SBR) to ensure adhesion. A skim over emulsion without proper bonding will delaminate.

How thick should a skim coat be?

3-5mm in total. Anything above 5mm is likely to crack as it dries; anything below 2mm may dry too fast before you can work it and will show marks. If the surface is very uneven and requires more build, use a base coat (Hardwall or Bonding) first, allow to set, then skim.

Can I use any brand of finish plaster?

British Gypsum Thistle MultiFinish is the industry standard in the UK, but other brands (Knauf, Lafarge) produce compatible products. Do not mix brands or products — different gypsum formulations may have incompatible setting times and strengths.

How do I stop plaster from setting too fast?

In hot, dry conditions: cool the mixing water (add a few ice cubes), dampen the surface before applying, and work in smaller sections. If the room is very hot and dry, consider working in the evening when temperature drops.

Regulations & Standards

  • BS EN 13279-1 — gypsum binders and plasters; classification and requirements

  • BS EN 13279-2 — test methods for gypsum plasters

  • Building Regulations Approved Document B — plaster specification for fire-rated walls (gypsum plaster contributes to fire protection on plasterboard systems)

  • British Gypsum White Book — comprehensive specification guide for all Thistle products (not a regulation, but the industry reference standard)

  • British Gypsum Thistle Technical Guidance — official application guides for all Thistle products

  • Knauf Plaster Application Manual — alternative product guidance

  • CITB Plastering NVQ Guidance — industry training standards

  • plasterboard types — choosing the right board before skimming

  • dot and dab — when dot and dab is the preferred fixing method

  • interior emulsion — mist coating and painting new skim correctly

  • damp stain blocking — treating stains on old plaster before rescimming