How to Repair Large Holes in Exterior Masonry
Large holes and damage in exterior masonry need a tough, weather-resistant repair to hold up long term. Here's how to get a solid, lasting result using the right products and the right process.
What counts as a large hole in exterior masonry?
Any hole or area of damage deeper than 10mm or wider than a standard crack. This includes impact damage to brick or block, holes left by removed brackets, pipework or fixings, or frost-damaged masonry, and areas where render has broken away exposing the substrate beneath. These repairs need a filler that can handle depth, resist moisture, and cope with the movement that comes with being permanently exposed to the elements.
Step 1: Prep - Clear, Clean, and Stabilise the Surface
Start by removing all loose, damaged, and crumbling material from the hole and the surrounding area. Use a cold chisel and hammer to cut back to solid masonry. The aim is a clean, stable edge with no crumbly material left behind - anything loose will compromise the adhesion of the filler.
Brush out all dust and debris thoroughly.
If the substrate is powdery, crumbly, or badly weathered, apply Wall Hardener before filling. Wall Hardener penetrates and consolidates weak substrates, giving the filler a solid base to grip, giving a more durable fill. Apply liberally using a brush, working it into all exposed surfaces inside the hole and around the edges. Allow to dry for a minimum of 24 hours before filling. This will not compromise the breathability of the substrate as Wall Hardener is vapour permeable.
Step 2: Fill - Apply the Filler
Applying exterior filler
Load your filling knife with your chosen filler, and press the filler firmly into the hole. Work from the inside out. The blade needs to be angled to ensure you get all the material onto the surface without leaving any air pockets. Air trapped behind the surface is the main cause of hollow spots and cracking once the filler has set.
Build the filler up to the level of the surrounding masonry and scrape off any excess with your filling knife held at a close angle. Aim to finish flush with the surface - overfilling creates unnecessary work at the finishing stage.
Depending on the exterior filler you've chosen, each will behave differently once applied. Rock Solid Repair Filler dries incredibly strong, so it's a good idea to blade off any excess and shape it to the final form before it sets. Tough Multi-Filler, Masonry Repair Filler and Quick Fill Exterior can all be sanded, but as with any exterior filler, they're hard-wearing, so test on a small area first to check how easily it sands.
Allow the filler to set fully before moving on to the finishing stage. Check the packaging for guidance for setting times, and bear in mind that cold or damp conditions will extend them, and warm dry conditions will accelerate them.
At this point, the repair is complete. All exterior fillers should be overcoated with a suitable water-based masonry paint to make them weatherproof, so even without a finish coat, the repair painting needs.
Step 3: Finish - Shape and Smooth for Decoration
Now the repair is complete if you want to enhance the finish and achieve a professional, paint-ready result, apply your chosen finishing filler over the repair and surrounding surface, and feather out the edges to blend seamlessly into the surrounding surface.
We advise all fillers should be overcoated with a suitable water-based exterior masonry paint, or similar, to make them weatherproof.