Float coat (cement render)
Float coat is the second of three coats in a cement render system: 6-10 mm thick, floated to a true plane, keys for the finish coat. Also called the brown coat.
Ask Chalkline about this →A float coat is the second (middle) coat of a three-coat cement render system. It sits between the scratch coat (first coat, keys to the substrate) and the finish coat (third coat, the visible surface). The float coat is typically 6-10 mm thick and is screeded to a true plane using a long aluminium or timber rule, then floated with a wooden or sponge float to close the face and create a key for the finish coat. The float coat is also called the brown coat in some traditions.
Position in the three-coat system:
| Coat | Thickness | Purpose |
|---|---|---|
| Scratch (first) | 4-8 mm | Bonds to substrate (block, brick, fibre cement), scratched horizontally to key the next coat |
| Float (second) | 6-10 mm | Brings the surface to a true plane; floated to close the face and key for finish |
| Finish (third) | 2-5 mm | Decorative surface; trowelled smooth, textured, or float-finished |
Total system thickness: 12-23 mm.
Float coat procedure:
- Wait for scratch coat to set (typically 1-3 days; light surface dryness, still damp inside).
- Mist the scratch coat before applying float to reduce suction.
- Apply the float coat using hawk and trowel; build up to required thickness.
- Screed off with a long straight edge to bring to a true plane.
- Float with wooden or sponge float to close the surface and create the key.
- Cut horizontal scratch lines lightly with a scarifier or comb (1-2 mm deep) to provide key for finish coat.
- Cure under cover and damp conditions (typically 2-3 days before finish coat).
Why the float coat matters:
- Trueness of plane: this coat sets the final wall plane that the finish coat will follow.
- Key for finish coat: the floated surface bonds the finish coat; smooth or polished surface fails to bond.
- Strength of the system: thickness here gives the system its mechanical strength.
- Crack control: thinner float coats are more prone to crazing; thicker (6-10 mm) within recommended range.
Material mix (typical for residential cement render float coat):
| Component | Ratio (by volume) |
|---|---|
| Cement | 1 |
| Lime (hydrated builders lime) | 1-2 |
| Sand (washed plastering grade) | 5-7 |
| Water | To workability |
Some renderers add plasticisers or fibre admixtures. Polymer-modified renders (e.g. Rockcote, Dulux Acratex) come premixed.
Common builder issues:
- Skipping the float coat (two-coat system instead of three): saves time but loses thickness and crack control.
- Float too thin (<5 mm): more prone to crazing.
- Float too thick (>12 mm): prone to slumping during application; surface drying issues.
- Smooth-trowelled float (no key): finish coat doesn’t bond.
- Insufficient cure time: finish coat applied too soon; bonding issues, drying shrinkage transfer.
- Wrong sand grade: render too rich or too lean; cracking.
For builders:
- Spec a three-coat render system for any structural masonry or block external rendering: don’t accept two-coat where AS 3700 expects three.
- Confirm the renderer keys the float coat with light horizontal scratches: a smooth-trowelled float coat will lose the finish coat.
- Allow proper cure time between coats: 2-3 days minimum per coat in normal conditions.
- Photograph each coat during application: useful for warranty claims if cracking emerges later.
- Don’t render in direct sun or wind: rapid drying causes crazing; render in shaded conditions or with sun shading.
Also known as: brown coat, second coat render, middle coat render.
Category: Render / plastering / finishes.
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Last updated: 2026-05-16. Verified: 2026-05-16.