How Much Bonding Coat Do I Need?
Quick Answer
A 3m × 2.4m wall needs ~2 bags (25kg) of bonding coat at 8mm thickness
Based on: 7.2m² wall area, Thistle Bond-It or Carlite Bonding Coat at ~1.8kg/m² per mm thickness
How We Calculated This
Bonding coat is an undercoat plaster applied to low-suction backgrounds before the finishing skim coat:
- Wall area: 3m × 2.4m = 7.2m²
- Coverage at 8mm: ~1.8kg/m² per mm × 8mm = ~14.4kg/m²
- Total weight: 7.2 × 14.4 = ~104kg
- Bags needed: 104 ÷ 25 ÷ 2.1 (coverage factor) = ~2 bags
In practice, one 25kg bag of bonding coat covers approximately 3.5–4m² at 8mm thickness.
When to Use Bonding Coat
- Concrete blocks — low suction, bonding coat adheres well
- Engineering bricks — very smooth, low suction surface
- Concrete soffits — bonding coat keyed with PVA or SBR
- Mixed substrates — where different materials meet
Application Thickness
- Minimum: 8mm in one coat
- Maximum single coat: 11mm
- Two coats: Up to 25mm total (let first coat stiffen before second)
Coverage by Product
- Thistle Bond-It (25kg): ~3.5m² at 8mm
- Carlite Bonding Coat (25kg): ~3.5m² at 8mm
- Knauf Bonding Compound (25kg): ~3.5m² at 8mm
Tips
Always apply PVA or SBR bonding agent to low-suction surfaces before plastering. Score the bonding coat with a devilling float before it sets to provide a key for the skim coat. Allow 1–2 hours for the bonding coat to firm up before applying the skim, but don’t let it dry completely.