A float finish is one of the most rewarding ways to complete interior lime plaster walls — it produces a lightly textured, breathable surface that suits heritage homes, natural builds, and anyone wanting an authentic plaster look. The key to success is understanding lime's working properties: it sets slowly, needs moisture management, and rewards patience over rushing.
What Is a Float Finish?
A float finish is created by working the surface of a freshly applied or dampened lime plaster coat with a float — a flat tool that compresses and textures the surface simultaneously. Depending on the float type you use, you can achieve anything from a fine, almost smooth texture (sponge float) to a more pronounced open grain (wooden or plastic float). It differs from a steel-trowelled finish, which closes the surface right up and reduces breathability — something you generally want to preserve with lime.
Tools and Materials You'll Need
- Lime finish coat plaster — use a pre-mixed NHL (natural hydraulic lime) or air lime finish coat, or mix your own from lime putty and fine washed sand (typically 1:2.5 ratio by volume). Check with your supplier for the right mix for your substrate.
- Sponge float (for a fine texture) and/or a plastic or wooden float (for a more open texture)
- Steel finishing trowel (for initial spreading)
- Hawk (to hold your plaster)
- Spray bottle or garden sprayer filled with clean water
- Bucket, mixing paddle, and drill
- Plastic sheeting to protect floors and joinery
- Scratched or keyed base coat already applied and at the correct stage of cure
Preparing the Base Coat
Before you apply your finish coat, the scratch coat (and any intermediate brown coat) must be firm but not fully carbonated. For lime plaster, this typically means waiting at least a few days to a week between coats, depending on temperature and humidity — lime does not like being rushed. The base coat surface should be scratched (keyed) to give the finish coat something to grip.
Dampen the base coat with a light mist of water before applying the finish coat. This slows suction and prevents the finish coat from drying out too fast, which can cause cracking. In New Zealand's varied climate — particularly in draughty older homes or during hot nor'westerly conditions — managing moisture is critical. Avoid working in direct sun or strong wind without protection.
Mixing the Finish Coat
If mixing from scratch, combine lime putty (or bagged finish lime) with fine washed sand and enough water to achieve a smooth, creamy consistency — similar to thick yoghurt. Mix thoroughly and allow to slake (rest) for at least 10–15 minutes before use. Pre-mixed products are available from specialist lime suppliers and some Placemakers or Mitre 10 branches — always follow the manufacturer's instructions.
Applying the Float Finish: Step by Step
1. Apply the Finish Coat
Using your steel trowel and hawk, apply the finish coat at approximately 3–5 mm thickness. Work in manageable sections — roughly 1–1.5 m² at a time — so the plaster doesn't dry before you can float it. Spread evenly with firm, sweeping strokes.
2. Allow to Firm Up
Wait for the plaster to reach the "green" stage — firm to the touch but not yet dry. Press your finger lightly into the surface; it should leave a slight impression without sticking. This is your floating window, and it varies with conditions.
3. Float the Surface
Using a damp sponge float, work in circular or figure-eight motions with light, even pressure. The float will pull up fine aggregate and create a consistent texture. Rinse and wring the sponge float frequently to keep it clean and slightly damp. For a coarser texture, use a wooden or plastic float with straight, sweeping strokes.
4. Mist and Re-float if Needed
If the surface starts to dry too quickly, lightly mist with water and continue floating. Avoid over-wetting — you want to extend the working time, not saturate the plaster.
5. Feather Joins
Where sections meet, blend carefully to avoid visible lines. Work wet-into-wet where possible.
6. Curing
Once finished, protect the wall from rapid drying for at least 48–72 hours. Keep windows partially closed, avoid direct heat sources, and mist lightly once or twice a day if conditions are dry. Lime cures through carbonation (absorbing CO₂ from the air), not just drying — this process continues for weeks.
Common Mistakes to Avoid
| Mistake | Why It's a Problem | Fix |
|---|---|---|
| Applying over a bone-dry base coat | Finish coat dries too fast, cracks | Pre-dampen the base coat |
| Floating too early | Plaster smears and drags | Wait for the green stage |
| Floating too late | Surface too hard to texture | Mist lightly and try again |
| Using a dirty float | Drags and tears the surface | Rinse frequently |
| Working in hot, windy conditions | Rapid drying causes cracking | Shade the area, mist regularly |
Finishing and Decoration
Once fully cured (allow several weeks before painting), a float-finished lime wall can be left natural, limewashed, or painted with a breathable paint such as a mineral or silicate paint. Avoid standard vinyl or acrylic paints, which can trap moisture and cause the plaster to fail. Resene offers breathable paint options suitable for lime substrates — check with their technical team for the right product.
When to Call a Professional
Lime plastering is a skilled trade. If you're working on a heritage or character home, dealing with large areas, or unsure about the condition of your existing base coats, it's well worth engaging an experienced renderer. Poor application can lead to delamination, cracking, or moisture issues that are costly to fix. A qualified renderer will also know how to assess your substrate and select the right lime system for your specific NZ conditions. You can find experienced local renderers on tradedirectory.co.nz.
For work on older homes, it's also worth checking with BRANZ or your local council if any consents are required — most interior plastering is exempt under Schedule 1 of the Building Act, but always confirm with your council.
Licensed Building Practitioner (LBP)
