What Is a Bonding Coat?

A bonding coat is a gypsum-based undercoat plaster designed for use on low-suction or smooth surfaces such as concrete, engineering bricks, and dense blockwork. In the UK, the most common bonding coat is Thistle Bond-It by British Gypsum.

When to Use Bonding Coat

Bonding coat is used when the background surface has low suction (doesn't absorb much water). Standard browning plaster would struggle to adhere to these surfaces, but bonding coat contains additives that help it grip. Common backgrounds include:

• Concrete walls and ceilings
• Dense concrete blocks
• Engineering bricks
• Surfaces primed with PVA or SBR
• Mixed-suction backgrounds (patches of different materials)

Application

Bonding coat is applied at 8–11 mm thickness in one or two coats. The surface is scratched with a devilling float to create a mechanical key for the finishing coat. Once set (but not necessarily fully dry), a skim coat of multi-finish plaster is applied over the top to give a smooth, paint-ready surface.

Coverage

A 25 kg bag of bonding coat covers approximately 2.5–3.5 m² at 8–11 mm thickness. Working time is approximately 1–1.5 hours from mixing.

Bonding vs Browning

The key difference is suction. Browning (Thistle Browning) is for high-suction backgrounds like common bricks and lightweight blocks. Bonding is for low-suction backgrounds. Using the wrong type can cause adhesion failure.