Question 1

What is bonding coat plaster?

Accepted Answer

Bonding coat (sold under brand names such as Thistle Bonding Coat or British Gypsum Bonding) is a gypsum-based undercoat plaster used on low-suction or non-absorbent surfaces where standard browning plaster would not adhere or dry correctly. Browning plaster requires a porous, absorbent background (brick, blockwork, aerated concrete) to draw water out of the plaster mix and allow it to set; on dense concrete, engineering brick, or painted surfaces, browning would not adhere and would slump. Bonding coat uses a different chemistry that does not require high background suction - it bonds mechanically to low-suction surfaces and dries more slowly under its own steam rather than by suction from the background.

Question 2

When should bonding coat be used instead of browning?

Accepted Answer

Bonding coat is the correct undercoat plaster for: dense concrete (walls, ceilings, soffits); dense concrete blockwork (not aerated or lightweight blocks); engineering brickwork (blue bricks, dense fired bricks); smooth or painted existing plaster surfaces being re-plastered; plasterboard (when a thick coat is needed rather than just a skim); EPS or PIR insulation board (using a polymer-modified version of bonding suitable for rigid insulation); and any surface with low suction or where browning plaster would not adhere. Browning plaster is used on high-suction backgrounds: aerated concrete blocks, medium density blockwork, traditional brick, and similar absorbent surfaces.

Question 3

How is bonding coat applied?

Accepted Answer

The background surface should be clean, sound, and free of dust, grease, and loose material. A PVA bonding agent (diluted 1:4 with water for the first coat, 1:1 for the tack coat) is applied to the surface and allowed to become tacky - do not apply plaster onto wet PVA, only onto tacky PVA. The bonding coat is mixed to a firm but workable consistency and applied with a plastering trowel to a maximum single-coat thickness of approximately 11mm. If more thickness is needed, a second coat can be applied after the first has stiffened. The surface of the bonding coat is scratched (keyed) with a scratch comb while still green (before final set) to provide a mechanical key for the finish coat. The bonding coat is allowed to set and firm up (usually 24 hours minimum) before the 2mm finish skim coat is applied.

Question 4

What is the difference between bonding coat and browning?

Accepted Answer

Both bonding coat and browning are gypsum-based undercoat plasters, but they are formulated for different backgrounds. Browning plaster requires a high-suction, absorbent background - the suction from the background draws water out of the plaster and accelerates set. It is suitable for brick, lightweight block, and similar porous surfaces. Bonding coat does not require suction from the background - it sets through its own hydration chemistry and adheres to low-suction or non-absorbent surfaces by mechanical and chemical bond. Using browning on a low-suction surface results in the plaster sliding off the wall before it can set; using bonding coat on a high-suction background results in it setting too quickly and unevenly. The background determines which product to use - when in doubt, apply PVA first and use bonding coat.

Bonding Coat

Just so you know...