Render Preparation: What Happens Before the Finish Coat

Why the finish name tells you so little

Two quotations for silicone or monocouche can describe very different jobs. One may assume clean blockwork handed over by a builder; another may include stripping failed render, repointing and dubbing out an uneven wall. The finish is the last few millimetres; the preparation decides whether they stay on.

A monocouche, for instance, is designed for clean, sound masonry free of anything that would stop it bonding, according to K Rend's data sheet for its K Mono. Getting an old wall to that state can be more work than applying the render itself.

So read the preparation lines first. Words such as prepare as necessary cover anything from brushing down to rebuilding parts of the wall.

Monocouche rendering ↗

Stripping and what it reveals

Failed render is hacked off down to the brick or block, working from the top. SPAB's advice on removing cement from old walls is to work with hammer and chisel rather than levering off large sheets, which can damage soft brick and stone. What lies behind is only known once it is exposed: good brick, poor brick, old repairs, filled openings, timber or concrete lintels.

Areas that could not be reached at the survey, such as high walls or wall behind fittings, should be inspected as soon as the scaffold is up, before the main stripping starts.

Repairing the masonry

Loose and crumbling joints are raked out and repointed. Spalled or cracked bricks are cut out and replaced. Cracks through the masonry are investigated before they are covered, because a crack caused by movement will come back through new render. In a former coalfield, a stepped or widening crack is also a reason to pause and get a chartered surveyor's view, as GOV.UK's subsidence guidance suggests.

Dubbing out and levelling

Stripped walls are rarely flat. Deep hollows, missing brick faces and steps between old and new masonry are filled with a dubbing-out coat before the main base coat, so the main coats can be laid at an even thickness. Thin systems are less forgiving: a silicone finish follows any unevenness in the coat below it, and INCA notes that thin basecoats over insulation need a very flat surface.

Suction, beads and mesh

Very absorbent brick draws water out of fresh render before it can set properly, which weakens the bond. K Rend's instructions advise damping down walls before rendering to control suction. In hot weather they suggest starting on the shady side and following the sun round.

Beads are fixed before the coats: a bell-cast bead above the DPC, angle beads at corners, stop beads at frames and brick. Mesh is bedded into the base coat where the background changes, for example over lintels or where old brick meets new block, to reduce the risk of cracks following the joint.

Each of these items is small on its own, but together they explain why two walls of the same size can take very different amounts of time to prepare.

Curing and waiting

Every coat needs time before the next. K Rend's base coats are normally left 24 hours; its silicone finish needs the base to stand at least 14 days; its primer is given 24 hours to dry. Lime coats can take about a week each. Those waits keep the scaffold up and are part of the preparation, even though nothing visible happens. Weather adds to them: most cement-based products are not applied below 5°C or with frost forecast.

When the plan changes

Sometimes stripping reveals more repair than anyone expected. The useful response is to stop, photograph what was found, explain the extra work and agree it before carrying on. A quotation that sets out how that will be handled is more helpful than one that assumes nothing will be found.

For example, on a hypothetical rear wall that looked sound from the garden, stripping might reveal an old doorway filled with mixed bricks and a steel lintel above a window with flaking rust. The filled doorway would be tied in and covered with mesh in the base coat; the lintel would be cleaned and treated or replaced by the right trade before rendering. Neither could have been priced with confidence before the render came off.

What poor preparation looks like later

Most render failures trace back to preparation. Render laid over loose material or dust comes away in sheets, often sounding hollow within a year or two. Render laid too thinly over uneven walls cracks along the high points. A wall that was not damped down on a hot day can leave a weak, powdery bond. Joints between old brick and new block that were not reinforced show as straight cracks.

At the base, render taken down over the DPC, or left without a bell-cast bead, stays wet and blows. Around openings, missing stop beads and sills without drips leave ragged edges and stains. None of these faults shows on the day the scaffold comes down, which is why the preparation record matters.

If you are comparing quotations, ask each contractor what they expect to do to your walls before the first coat, wall by wall. The answers usually say more about the job than the finish name does.

The preparation record

Once the finish is on, the preparation is invisible. Photographs of the stripped wall, the repairs and the beads, with a note of the products used, let a future repairer understand what is behind the render. The Rotherham rendering homepage links the services where this preparation applies.