Repairing Cracks in Render Before Painting

Start with the cause, not just the crack

Before you reach for filler, take a step back and ask why the render has cracked. Render is a sacrificial skin. It is meant to shed water and protect the masonry beneath. When a crack appears, water can track sideways, freeze, expand and quietly blow the render away from the wall. That is why filling cracks is not just cosmetic. A good repair stops water ingress and buys years of life for the wall.

Look at the crack pattern. A few hairline cracks across a large panel are usually shrinkage or thermal movement. They are common on new render and on cement-rich mixes. Diagonal cracks from the corners of windows or doors often point to movement at an opening. Horizontal or stepped cracks may follow brick courses and can suggest lintel, wall tie or foundation movement. If the render sounds hollow when tapped, the bond has failed and filling alone will not last.

• Hairline cracks up to about 1mm: often cosmetic, but still worth sealing before painting.
• Wider static cracks 1–5mm: fill and reinforce with mesh.
• Live or movement cracks that open and close: use a flexible sealant, not a rigid filler.
• Structural cracks that are wide, stepped or accompanied by bulging: get professional advice before decorating.

Tools and materials worth having ready

You do not need a van full of kit, but using the right products makes the difference between a repair that lasts a winter and one that lasts a decade. Choose materials that are compatible with your render. Cement render takes cement-based or acrylic fillers. Lime render needs a lime-based filler so it can breathe and move in the same way. Silicone and acrylic renders need a flexible, compatible filler and sealant.

• A wire brush, pointing trowel, filling knife and a small club hammer and chisel for raking out.
• Exterior-grade masonry filler, or a sand and cement mix for deeper repairs.
• Flexible acrylic or MS polymer sealant for movement cracks.
• Alkali-resistant mesh tape or fibreglass scrim, about 50mm wide.
• Clean water, a spray bottle, a mixing board and a sponge.
• Breathable masonry paint and a suitable primer or stabilising solution.

Prepare the crack properly

Preparation is where most DIY render repairs fail. A dusty, crumbly crack gives filler nothing to grip. Rake out the crack to a shallow V shape, roughly 5mm wide and 5mm deep, using a filling knife or trowel. Remove all loose render, dust and old paint from the edges. Brush out the dust, then rinse with clean water and let the surface dry until it is damp but not wet. Dampening the render stops it sucking the water out of the filler too quickly.

If the crack is deep, fill it in layers rather than one thick dollop. For lime render, use a lime mortar mix and avoid cement, which is harder and less breathable. For cement render, a 3:1 sand and cement mix with a plasticiser works for deep pockets, but a ready-mixed exterior filler is easier for shallow cracks. Never repair in frost, rain or blazing sun. Ideal conditions are dry, mild and cloudy.

Fill, mesh and reinforce

For cracks wider than a hairline, reinforcement is the secret to a lasting repair. Press filler into the crack with a filling knife, then bed a strip of alkali-resistant mesh tape over the top while the filler is still wet. The mesh should bridge the crack by at least 25mm on each side. Cover it with a second thin layer of filler and feather the edges smooth. This creates a reinforced band that resists future movement.

For movement cracks, do not use rigid filler. Instead, rake out the crack, insert a foam backing rod if it is deep, then apply a low-modulus flexible sealant. Smooth it with a dampened tool and leave it slightly recessed so it can stretch. For hairline cracks that are too fine to fill, a flexible masonry paint with added fibres can bridge them, but only if the render is sound and dry.

Sealant, paint and weatherproofing

Once the filler is fully cured, usually 24 hours in good conditions, you can paint. Use a breathable masonry paint rather than a waterproof gloss or plastic coating. Breathable paint lets trapped moisture escape, while a sealed surface can trap damp and cause it to blister or flake. If the render is very porous, apply a stabilising primer first. Then apply a mist coat diluted with 10–20% clean water, followed by two full coats.

While you are up there, check the usual suspects for water ingress. Clear gutters and downpipes. Check flashings, window sills, reveals and door thresholds. Reseal around window and door frames with a compatible frame sealant or low-modulus silicone. Do not bridge the damp-proof course or block ventilation gaps. A cracked render is often a symptom of water arriving from somewhere else.

When to stop and call in a specialist

Some cracks are beyond a tube of filler. If a crack is wider than 5mm, continues to grow, runs diagonally across a wall, or is accompanied by bulging, crumbling masonry or damp patches inside, call a chartered surveyor or experienced render specialist. They can check for wall tie failure, lintel movement, foundation issues or failed damp-proofing. Repairing the render without fixing the cause is like painting over a leak: it may look good for a season, but the damp will return. A proper repair, with the right filler, mesh and breathable finish, will keep your walls dry and your paintwork sound for years.