How to Fix Reappearing Cracks in Masonry
Why Do Exterior Masonry Cracks Keep Coming Back?
Exterior masonry is constantly on the move. Temperature swings cause brick, block, stone and render to expand and contract. Moisture levels shift with the weather, the ground beneath the building settles over time, and traffic vibration adds to it too, especially on homes near busy roads. These forces never really stop, so the masonry never really stops moving either.
That's why previously applied filler often fails. If it's set hard rather than staying flexible, it eventually cracks again under the same forces that caused the problem in the first place. A flexible filling system bonds to both sides of the crack and stays workable, so it moves with the masonry instead of fighting it.
Important: Flexible fillers are for cosmetic, non-structural cracks only. If a crack is wider than 5mm, growing, or appears alongside sticking doors or windows, get a structural engineer or surveyor to check it before filling.
Step 1: Prep - Clean Out and Stabilise
Start by removing all old filler from previous repairs. If the previously applied filler has failed, it needs to come out completely. Use a chisel, scraper, or multi-tool to get back to a clean, stable edge on both sides of the crack, and clear out any loose masonry, mortar, and debris.
Clean off any dirt, moss, or algae in and around the crack. The surface needs to be clean and dry before filling.
Test the substrate before filling. Rub your hand or a dark cloth across the area around the crack: if it comes away dusty or powdery, or the surface crumbles under light pressure, the substrate is failing and needs treating.
Apply Toupret Wall Hardener, working it into both sides of the crack and the surrounding surface, and leave it to dry for at least 24 hours before filling.
Step 2: Fill - Apply Flexible Filler
Fill-Flex is a ready-mixed flexible filler that stays permanently flexible after setting. It absorbs substrate movement rather than cracking under it, which makes it the right choice for cracks that keep coming back.
Using a filling knife, apply Fill-Flex firmly into the crack, working it in from the outside edges. Press hard enough so the filler fills both sides and eliminates any air pockets or voids.
Smooth the filler to a flush finish as you apply it. This matters. Due to its flexibility, Fill-Flex is resistant to sanding once set, so the surface needs to be as smooth and level as possible during application.
The crack is now repaired, and it must be overcoated with paint to make it waterproof.
Step 3: Finish - Flexible Skimcoat
Once Fill-Flex has fully set, apply Skim-Flex over the repair and feather it out across the surrounding surface. Skim-Flex keeps its flexibility after drying, so the whole repair system, from fill to finish, moves with the substrate without cracking.
Apply with a filling knife or blade, working it to a thin, even coat. Feather the edges out well beyond the crack line to blend the repair into the surrounding wall. If the masonry has a textured finish, shape the Skim-Flex while it's still setting to match the profile.
Let it dry fully. The repair is now ready for an exterior paint system. Because both Fill-Flex and Skim-Flex stay flexible once dry, the finished surface will keep absorbing movement without cracking, giving you a permanent repair.