What is the coverage of Thistle MultiFinish at 13mm?

Thistle MultiFinish at 13mm thickness covers approximately 1.6-2.0 m² per 25kg bag depending on the background surface. On medium-suction blockwork, expect around 1.8 m²/bag. On high-suction brick, coverage drops to approximately 1.6 m²/bag. When used as a 2mm skim coat over plasterboard, one bag covers 8-10 m². These rates are from British Gypsum's published product data.

When should I use Hardwall vs MultiFinish?

Thistle Hardwall is an undercoat plaster for high-suction backgrounds like brick, block, and aircrete — it provides a strong, flat base at 11mm thickness. MultiFinish is versatile and can be used as both a one-coat plaster at 13mm on most backgrounds OR as a 2mm skim finish coat over Hardwall or plasterboard. For solid walls, many plasterers use Hardwall as undercoat then MultiFinish as the skim finish.

How does background type affect coverage?

High-suction backgrounds (brick, aircrete blocks) draw water from the plaster faster, requiring a thicker application and reducing coverage by 10-20%. Low-suction backgrounds (plasterboard, concrete) allow thinner, more consistent application. Always dampen high-suction backgrounds or apply PVA/SBR bonding agent before plastering to control suction and improve adhesion.

How much water do I need per bag of plaster?

Most 25kg bags of plaster require approximately 10-11 litres of clean, cold water. Always add the plaster to the water (not water to plaster) and mix thoroughly until smooth. The mix should be workable but not runny. In warm weather, use cold water to extend the working time. Plaster typically has a working time of 60-90 minutes once mixed.

Can I apply MultiFinish at more than 13mm thickness?

British Gypsum recommend a maximum 13mm application thickness for MultiFinish in a single coat. For deeper builds (e.g. on very uneven walls), apply Thistle Bonding Coat or Hardwall first to level the surface, then skim with MultiFinish at 2mm. Applying too thick in one coat risks cracking, slow drying, and a weak finish.

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One Coat Plaster Calculator — MultiFinish vs Hardwall Coverage

Calculate plaster bags needed for one-coat application. Compare Thistle MultiFinish at 13mm vs Hardwall at 11mm across different background types.

Wall Area

Total area to be plastered

Plaster Product

Select which plaster you are using

Background Type

Affects coverage rate by 10-20%

Price Per 25kg Bag (optional)

Enter your supplier price per bag

How We Calculate This

This calculator estimates the number of 25kg bags of one-coat plaster needed for your wall area. It uses British Gypsum's published coverage rates for the Thistle range, adjusted for different background surface types.

The formula

Bags needed = (Wall area ÷ Coverage per bag) × Wastage factor

Coverage per bag varies significantly by product and background. MultiFinish on plasterboard at 2mm covers 8-10 m²/bag, but the same product at 13mm on blockwork only covers 1.8 m²/bag. Hardwall at 11mm covers 2.0-2.5 m²/bag depending on the substrate. High-suction surfaces always reduce coverage.

Key coverage rates

MultiFinish on plasterboard (2mm): 9.0 m²/bag
MultiFinish on blockwork (13mm): 1.8 m²/bag
Hardwall on blockwork (11mm): 2.2 m²/bag
One Coat on blockwork (13mm): 2.2 m²/bag

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.