How to render exterior walls

Video by Skill Builder. This walk-through draws on the video "How to Render Exterior Walls - Beginner Tips" from Skill Builder, presented by Roger Bisby. He revisits earlier rendering advice with the benefit of professional feedback, covering the scratch coat, how to check flatness, and the right mix ratios. Worth watching before you open a bag of sand.

1. Assess the wall and sort the prep

Before you touch a trowel, check what you are working with. Old lime render needs to come off if it is hollow or crumbling — tap it with your knuckle and listen for a dull sound. Solid areas can usually stay. Loose mortar joints should be raked back and repointed before you render over the top, otherwise the render will crack along those lines.

Give the wall a good brush down with a stiff wire brush to remove dust, moss, and any loose material. If there is significant algae or mould, treat it with a diluted fungicidal wash a day or two beforehand and let it dry out. On very porous surfaces like old brick or lightweight block, prime with a diluted SBR bonding agent — one part SBR to four or five parts water. It makes a real difference to how well the first coat grips.

2. Mix your scratch coat correctly

A standard sand and cement scratch coat is usually mixed at around 4:1 or 5:1 sharp sand to cement, depending on the suction of the wall and the weather. Softer mixes are more forgiving on flexible or mixed substrates; stiffer mixes suit smooth, low-suction surfaces. Add just enough water to make the mix workable — it should hold its shape on the hawk without slumping.

That said, pre-bagged renders like Weber or Parex take the guesswork out of mixing and tend to give a more consistent result, especially for larger areas. They cost more than mixing your own, but for a first-time renderer that trade-off is usually worth it. Whichever route you go, mix in small batches until you have the hang of the consistency.

3. Apply the scratch coat in even passes

Load your hawk, take a trowel-full of render, and apply it to the wall at roughly 45 degrees, pushing upwards and spreading in sweeping arcs. Aim for a thickness of around 8–10 mm on the first coat. Do not try to get a perfect finish at this stage — the scratch coat is purely there to give the top coat something to key to.

Work in manageable sections, especially on warm or windy days when the render can skin over quickly. Avoid rendering in direct sunlight if you can. Once a section is applied and has started to firm up (but not gone hard), drag a scratch comb or a length of serrated batten over it to leave horizontal grooves. These grooves are what hold the next coat in place — skip them and the top coat can delaminate months later.

4. Let the scratch coat cure properly

Leave the scratch coat to cure for at least 24 to 48 hours before applying the finish coat — longer in damp or cold weather. In warm weather, lightly mist the scratch coat with water if it is drying out too fast. The render needs moisture to cure correctly; if it dries out in a hurry it will be weak and the finish coat will not bond.

Do not be tempted to rush this stage. It is the most common mistake on rendering jobs. The surface should feel firm and slightly damp to the touch, not powdery or saturated, before you move on. If you are rendering an east-facing wall on a property in Sandwich or the villages around it, early morning sun can dry things out faster than you expect in summer.

5. Mix and apply the finish coat

The finish coat is typically a finer mix than the scratch coat — either a proprietary through-coloured render or a sand and cement mix at around 5:1 or 6:1 with a small amount of lime added for workability. Apply it at around 6–8 mm thick, working in consistent passes and keeping a wet edge so each new application blends into the last.

Use a straight-edged darby or a long feather-edged board to rule off the surface and check it is flat. Hollow areas can be filled at this stage while everything is still green. Once the coat has started to firm up — it should feel like firm cheese — go back over it with a plastic float in circular motions to close the surface and bring up a smooth or textured finish, depending on what you are after.

6. Protect and cure the finished render

In hot or dry weather, keep the finished render damp by misting it lightly once or twice a day for the first couple of days. In cold weather, protect it from frost with hessian sacking or render fleece — a sharp frost on fresh render will pull moisture out and leave the surface crumbly. Do not paint it until it has fully cured, which typically takes at least four weeks for sand and cement render.

Once cured, apply a masonry stabiliser before painting if the surface is powdery at all, then finish with a quality exterior masonry paint in at least two coats. Smooth render on exposed Kent coastline properties will need a flexible, breathable paint to cope with the weather. Mind you, even a well-rendered wall needs checking every few years — small cracks around windows and corners let water in fast.

When to call a handyman

Call Richard if the wall has significant structural cracks, if the existing render has failed across a large area, or if you are not confident about mixing and applying multiple coats to a consistent standard. Rendering above ground-floor height is also a job where having a second person makes it significantly safer and faster. Richard can handle the prep, application, and finishing on properties across Sandwich and the surrounding East Kent area.