Repairing Blown Render: Patching Holes Before You Paint

You are inspecting your exterior walls before painting. You see a hairline crack. You press on it, and the wall moves slightly. Or perhaps you tap it with your knuckles, and instead of a solid "thud," you hear a hollow, drum-like "clunk."

This is Blown Render. It means the cement skin of your house has de-bonded from the brickwork underneath, small areas like this might not be a huge problem, but if they spread you can end up having whole sheets of render stripping off your walls.

If you decide to paint over a large air pocket, the weight of the paint can often be enough to pull the entire patch off the wall, ruining your new finish and setting you back to square one.

You cannot glue it back. You have to remove it and replace it. Here is the step-by-step guide to patching exterior walls seamlessly.

1. The Diagnosis: The Hammer Test

You need to find the edges of the damage. Moisture gets behind the render, freezes, expands, and pops the cement off the brick (Frost Heave). This creates a pocket of air. You can usually tell by tapping these areas whether or not they have developed an air pocket underneath the render.

• The Tool: Use the handle of a hammer or a screwdriver.

• The Method: Tap around the suspect area.

• Solid Ring: The render is attached.

• Hollow Thud: The render is blown.

• Mark it: Use a piece of chalk to draw a circle around the entire hollow area. You might be surprised at how big it is.

2. The Demolition: Be Brave

This is the part DIYers hate. You have to make a hole in your house.

1. Hack it off: Use a hammer and bolster chisel (or a small SDS drill) to remove the loose material inside your chalk circle.

2. Find the "Sound" Edge: Keep chipping away until you reach render that is stuck fast to the wall. Do not leave "slightly loose" bits at the edges—they will crack later.

3. Square it off: Try to cut the edges of the hole square. Undercut the edges slightly (angle the chisel) to create a "dovetail" key for the new mortar to lock into.

4. Clean the Brick: Wire brush the exposed brickwork to remove old salts and loose debris.

3. The Secret Ingredient: SBR Bonding Agent

You cannot just slap fresh cement onto dry, dusty bricks. It will fall off. You need a bonding bridge. In the past, builders used PVA. Do not use PVA outside. It re-wets when it rains and rots the repair. You must use SBR (Styrene Butadiene Rubber).

• The Priming Mix: Mix 1 part SBR with 3 parts water. Brush this onto the exposed bricks and the edges of the old render.*

• The Bonding Slurry (Optional but Recommended): For a bulletproof bond, mix neat SBR with a handful of cement powder to make a sticky paste. Paint this onto the brickwork while wet.

*Always follow the instructions on the package for your specific product.

4. The Fill: Repair Mortar vs. Sand & Cement

Now you need to fill the hole.

Option A: Ready-Mixed Repair Mortar (Easier)

For small patches (less than a football size), buy a tub or bag of Polymer Modified Render Repair.

• It is lightweight, non-slump (won't slide down the wall), and cures fast.

• Just add water and trowel it in.

Option B: Traditional Sand & Cement (Cheaper)

For large areas.

• The Recipe: 4 parts Plastering Sand to 1 part Cement. Add a splash of Waterproofer/Plasticiser.

• Layering: If the hole is deep (>20mm), apply it in two coats.

• Scratch Coat: Fill halfway, scratch the surface with a comb, let it dry.

• Top Coat: Apply the final layer the next day.

5. The Finish: Blending In

The goal is to make the patch invisible once painted.

1. Overfill slightly: Put slightly more mortar in than needed.

2. Rule it off: Use a straight edge (piece of timber) to scrape the mortar flush with the existing wall.

3. The Sponge Float: Wait 20-40 minutes for the mortar to firm up (green hard). Take a damp sponge float and rub the surface in circular motions.

• This blends the edges of the new patch into the old wall.

• It brings the "aggregate" to the surface to match the sandy texture of the old render.

6. Drying Time: The Alkalinity Trap

Fresh cement is highly alkaline (pH 12+). If you paint over it immediately, the alkali will burn through the paint, causing "Saponification" (the paint turns into a soapy mess and slides off).

• Wait: Ideally, let the patch cure for 3-4 weeks.

• The Cheat: If you can't wait, use an Alkali-Resistant Primer before applying your topcoat.

Conclusion

A blown patch is a big problem for your wall. It will grow as more water gets trapped behind it. Cutting it out sounds drastic, but it is the only way to ensure that not only your expensive paint job lasts, but the render itself as well.

• Tap to find the hollows.

• Use SBR, not PVA.

• Blend the texture with a sponge.

Render secure and ready to start painting?

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