Rendering a first coat (also called a scratch coat or base coat) is the process of applying the initial layer of plaster or mortar to a wall surface. This base layer provides the foundation for subsequent finish coats. The term also relates to the setting or hardening of mortar — the point at which the mix begins to cure and can no longer be worked.
The scratch coat in plastering
In plastering and rendering, the first coat is applied directly onto masonry or concrete. It serves two key purposes:
1. Creating a bond — The rough texture of the base coat gives the finish coat something to grip onto
2. Levelling the surface — Irregularities in the masonry are filled and evened out
How to apply
- Dampen the substrate (dry bricks draw moisture out of the mortar too quickly)
- Apply the mortar using a plastering trowel or hawk and trowel
- Keep the surface rough — a smooth base coat reduces adhesion for the next layer
- Allow the base coat to cure for at least 24 hours before applying the finish coat
Thickness
The first coat is typically 8 to 12 mm thick. On smooth concrete a thinner coat may suffice; on rough or uneven masonry a thicker application is needed.
Setting and hardening of mortar
When mortar or concrete “sets”, the binder (cement) begins to react with water in a chemical process called hydration. Once setting has begun, the mortar can no longer be worked. Mixed mortar should therefore be used within 1 to 2 hours.
Factors that affect setting time:
- Temperature — Heat accelerates setting, cold slows it down
- Water content — Too much water delays setting and reduces final strength
- Cement type — Rapid-set cement hardens within minutes, standard Portland cement takes several hours
Application in DIY projects
When building a garden wall or rendering an outbuilding, the first coat is a critical step. Skipping it or applying it incorrectly leads to cracking, poor adhesion, and a finish coat that flakes off over time.
Related terms
- Mortar mix
- Masonry
- Joint
- Concrete
- Reinforcement
- Bond pattern
Learn more about building terms in our knowledge base at fredsdiyplans.com.