Domakihub How to Float and Set a Wall: A Complete Plastering Guide
Master the two-coat float-and-set method that gives masonry walls a smooth, paint-ready finish.
Reviewed by Domakihub home & decor pros · 2026-06-23 · 6 min read
Floating and setting a wall is the traditional two-coat plastering method used to bring bare brick or block masonry up to a smooth, durable interior finish. The video above walks through the full process on a real brick wall, while this guide breaks it into clear steps with the product knowledge and timing cues that make the difference between a professional result and a cracked or hollow finish. Whether you are renovating a stone villa on a Greek island, refreshing a holiday apartment in Athens, or refurbishing a hotel room, understanding this process helps you brief your plasterer accurately and spot quality issues before they are painted over.
What Float and Set Actually Means
Float and set refers to a two-stage plastering system applied to masonry backgrounds such as brick, concrete block, or stone. The first coat — the float coat — is a thick undercoat (typically sand-and-cement render or a pre-mixed undercoat such as Hardwall) that builds out the wall to a flat, even plane and provides mechanical strength. The second coat — the set coat — is a thin finish plaster (usually around 2-3 mm) that goes over the cured float and is trowelled to a smooth, hard surface ready for painting or wallpaper.
The key distinction from a single skim-coat job is that floating corrects significant irregularities in the masonry behind it. In older Greek properties with irregular stone or hand-laid brick, the float coat can vary from as little as 6 mm to over 20 mm in places, making it the right choice wherever the wall is uneven, highly absorbent, or previously unplastered.
When to Call a Licensed Professional
Applying a float-and-set finish is skilled work but does not require a permit on a standard interior partition. However, if the masonry shows signs of rising damp, efflorescence (white salt deposits), or structural cracking, you must have a structural engineer or licensed waterproofing specialist assess the wall before any plaster goes on. Plastering over active damp locks moisture into the structure, accelerates mould growth, and guarantees the new plaster will fail within a season — a common and costly mistake in coastal and island properties across Greece.
Any wall that forms part of a load-bearing structure, party wall, or fire compartment must be assessed by a civil engineer (Poleodomia-registered) before openings are formed or the wall is significantly cut into. If the renovation touches a column, beam, or pillar, Greek building regulations (N. 4495/2017 and its amendments) require a permit and engineer supervision regardless of whether plastering is involved.
How to Float and Set a Wall: Step by Step
- 1
Prepare and prime the masonry background
Chip away any loose mortar, old plaster, or paint that has lifted from the surface. Rake out open joints to a depth of about 10 mm so the undercoat has something to key into. Brush off all dust and debris, then apply a bonding agent or PVA diluted 1:4 with water and allow it to become tacky before applying the float coat. On very dense or low-suction surfaces such as engineering brick or dense concrete block, use a purpose-made bonding coat (e.g., Thistle Bond-It or equivalent) and scratch it to create a mechanical key. Skipping this step is the single most common cause of delamination.
- 2
Mix and apply the float coat
For a sand-and-cement float, a standard mix is 4 parts sharp sand to 1 part Portland cement with a small amount of plasticiser. Pre-mixed undercoats such as British Gypsum Hardwall or equivalent Greek-market products (e.g., Saint-Gobain Weber render systems) are more forgiving for beginners. Apply the mixed material firmly onto the primed wall with a hawk and a steel trowel, pushing it into the background and building up to a minimum of 8-11 mm in a single pass, or up to 20 mm in two passes if the wall is particularly uneven. Work from the bottom of the wall upwards in overlapping vertical sweeps.
- 3
Rule off and key the float coat
While the material is still workable (typically 20-40 minutes depending on ambient temperature and product), use a straight, rigid aluminium rule — called a darby or feather-edge rule — placed against the screeds or grounds you have set at each corner of the wall. Draw it upward in a sawing motion to strike off the high spots and fill the low spots. The goal is a flat plane, not a smooth one. Once the surface is flat and has lost its initial wet sheen but is still slightly tacky, scratch the entire face with a devilling float or a nail comb to create a cross-hatched key for the finish coat. Allow the float coat to cure for a minimum of 24 hours (48-72 hours in cooler or humid conditions, which are common in winter on Greek islands).
- 4
Apply the finish (set) coat
Mix your finish plaster (e.g., Thistle Multi-Finish or an equivalent gypsum-based product) to a creamy, lump-free consistency — it should flow off the trowel slowly. Dampen the float coat lightly with a clean brush or spray bottle so it does not pull moisture out of the finish too quickly. Apply the finish plaster in two thin passes: the first to cover the wall and fill any remaining hollows (roughly 1.5 mm), and the second laid immediately over the first to build up to approximately 2-3 mm total. Work systematically across the wall without stopping mid-panel, as join marks are very difficult to erase once the material starts to set.
- 5
Trowel to a smooth finish
As the finish coat begins to firm up — you will notice the sheen diminishing and the surface resisting the trowel lightly — begin the flatting-up process. Hold the steel finishing trowel at a very shallow angle (almost flat to the wall) and use long, overlapping arcs with firm, even pressure to consolidate the surface and close any small pinholes or trowel marks. Splash a small amount of clean water onto the surface with a brush or your fingertips if it is drying too quickly, then trowel again. Repeat two or three more passes as the plaster stiffens further, each time with slightly more trowel angle and pressure. The final result should be a hard, smooth, uniform surface with no hollows or ridges. Allow the set coat to dry fully — at least 7 days in summer Greek conditions, longer in winter — before applying any paint or primer.
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FAQ
- How thick should the float coat be on an old stone or rubble wall?
- On irregular stone or rubble masonry common in older Greek village houses and island properties, the float coat may need to be built up in two separate passes to avoid cracking from excessive thickness in a single application. As a rule, no single pass of sand-and-cement render should exceed 12-15 mm. If the wall requires more than 25 mm overall to bring it to plumb, it is worth setting timber or metal screeds at each corner and possibly mid-span to guide your rule. For particularly deep voids, pack with mortar or masonry filler first and allow to cure before rendering over.
- Can I plaster directly over painted masonry without stripping the paint first?
- Not reliably. Existing paint creates a weak interface between the masonry and the float coat, which will almost certainly lead to delamination within a few years — sometimes far sooner in the humid climate of coastal Greece. The correct approach is to wire-brush or mechanically scarify the painted surface, apply a suitable bonding agent, and if the paint is an oil-based or impermeable type, consider hacking the surface with a cold chisel to expose the raw masonry before priming. Testing adhesion first by applying a small float patch and attempting to pull it off after curing is a worthwhile check.
- How long does float-and-set plastering take to dry before I can paint it?
- The float coat (sand-and-cement or gypsum undercoat) needs a minimum of 24-48 hours before the finish coat is applied, and the finish coat (gypsum set) needs at least 7-14 days before painting in warm Greek summer conditions. In winter, cooler temperatures and higher humidity on islands such as Crete or Rhodes can extend drying times significantly — sometimes to 3-4 weeks for a full sand-and-cement system before it reaches the moisture content required for adhesion of emulsion paint. Always use a moisture meter rather than guessing by sight, and apply a water-based mist coat (diluted 10:1 emulsion) as a first layer to seal the porous surface before full-colour coats.