Complete Guide to Plastering

Types of Wet Plaster

UK plastering uses gypsum-based plasters manufactured to British Standard BS EN 13279. British Gypsum's Thistle range is the industry standard, with different products for undercoats and finishing coats.

Bonding Plaster (Undercoat)

Thistle Bonding Coat is an undercoat plaster for solid backgrounds with low suction, such as concrete blocks, engineering bricks, or painted surfaces. Applied 8-12mm thick, it provides a level base for the finishing coat. Coverage: 2.4m² per 25kg bag at 12mm thickness. Drying time: 1-2 hours between coats, 7-14 days full cure.

Finishing Plaster (Topcoat)

Thistle Board Finish or Thistle Multi-Finish are topcoat plasters providing a smooth, white surface for decoration. Applied 2-3mm thick over bonding coat or directly onto plasterboard. Coverage: 10m² per 25kg bag at 2mm thickness. Drying time: 2-5 days before decoration.

Multi-Finish Plaster

Thistle Multi-Finish is an all-purpose finishing plaster that works on most backgrounds, including plasterboard and bonding coat. This versatility makes it the most popular choice for plasterers. Same coverage and drying times as standard finishing plaster.

Browning Plaster

Thistle Browning is an undercoat for high-suction backgrounds like brickwork or blockwork. Applied 8-12mm thick, similar to bonding plaster but formulated for more absorbent surfaces. Often requires PVA sealer on very porous backgrounds.

Plasterboard vs Wet Plaster: Decision Tree

The choice between plasterboard and wet plaster depends on the substrate, project type, timeline and budget.

When to Use Wet Plaster

• Solid walls: Existing brick or blockwork requiring levelling or repair
• Curved surfaces: Bay windows, archways, Victorian cornices
• Heritage properties: Listed buildings where breathability is required
• Thick builds: Walls requiring more than 15mm levelling

When to Use Plasterboard

• New stud walls: Timber or metal frame partitions
• Ceilings: Faster and lighter than wet plaster lath systems
• Speed: Plasterboard can be skimmed same day, wet plaster requires undercoat drying time
• Insulation: Insulated plasterboard combines finish and thermal performance
• Sound proofing: Acoustic plasterboard for party walls or studios

What is the difference between bonding and finishing plaster?

Bonding plaster is an undercoat applied to solid backgrounds like brick or blockwork, typically 8-12mm thick. It provides adhesion and levelling for uneven surfaces. Finishing plaster is a topcoat applied over bonding or plasterboard, 2-3mm thick, providing a smooth white surface for decoration. You cannot apply bonding plaster without a finishing coat, and you should not apply finishing plaster directly to bare masonry (use bonding first, or PVA seal the wall).

Should I use wet plaster or plasterboard?

Use wet plaster for solid walls requiring levelling or repair, curved surfaces, or heritage properties where breathability matters. Use plasterboard for new stud walls, ceilings, faster installation, and when sound or thermal insulation is needed. Plasterboard with a skim coat is faster and lighter than traditional wet plaster systems. For DIY projects, plasterboard is easier to achieve good results, while wet plaster requires skill to mix and apply evenly.

Application Basics

Surface Preparation

All surfaces must be clean, dry and stable. Remove wallpaper, loose paint, dust and debris. High-suction backgrounds (brick, blockwork) require PVA sealer diluted 1:4 with water, applied 24 hours before plastering. Low-suction surfaces (concrete, painted walls) can be plastered directly with bonding coat or require mechanical key (scratching/scoring).

Mixing Plaster

Always add plaster to water, never water to plaster. For a 25kg bag of multi-finish, use approximately 11-12 litres of clean water. Mix for 2-3 minutes to a smooth, creamy consistency. Mixed plaster has a working time of 60-90 minutes. Never add water to plaster that has started to set.

Two-Coat Technique

Apply bonding coat 8-12mm thick using a laying-on trowel, pressing firmly to ensure adhesion. Scratch the surface with a devil float to provide key for the finishing coat. Allow to dry until firm but not fully set (1-2 hours). Apply finishing plaster 2-3mm thick, working in sections, and trowel smooth when firm. For plasterboard, one skim coat of 2-3mm is sufficient.

Drying Times

Gypsum plaster dries by evaporation, not chemical curing. Drying times depend on temperature, ventilation and humidity.

• Between coats: 1-2 hours (bonding to finishing coat)
• Before decoration: 2-5 days for skim coat, 7-14 days for full wet plaster system
• Full cure: 14-21 days for complete moisture evaporation

Do not paint over plaster until fully dry. Wet plaster appears darker pink, dry plaster is light pink or white. Mist coat (diluted emulsion paint) should be applied before full-strength paint to allow continued moisture release.

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