How to Fill Movement Cracks in Wood
Interior woodwork moves. Timber expands and contracts with changes in temperature and humidity, and where two pieces of wood meet, or where wood meets a wall, that movement opens up cracks and gaps over time. Here's how to fill movement joints in interior woodwork with a flexible system that stays intact.
Why do cracks appear at joints and junctions?
Wood is a natural material that responds to its environment. As humidity levels change through the seasons, timber absorbs and releases moisture, causing it to expand and contract. Central heating amplifies this, rooms go from cold and damp to warm and dry on a daily cycle, and the woodwork moves with it.
This movement is most visible where two pieces of wood meet at a joint, where wood meets a wall or ceiling, around door and window frames where the frame and the surrounding plaster move at different rates, and along skirting boards, architraves, and dado rails.
A rigid filler bonds to both sides of the gap. When the timber moves, the filler cannot flex with it and the bond breaks. A flexible filler absorbs that movement and keeps the joint sealed.
Step 1: Prep - Clean and Stabilize
Start by cleaning out the crack or gap. Remove any old filler, caulk, paint, or debris from the joint. If a rigid filler has failed previously, dig it all out, leaving it behind will prevent the flexible filler from adhering properly.
Use a scraper, filling knife, or utility knife to get back to clean timber and a stable edge on both sides of the gap. Brush out all dust and loose material.
If the timber around the joint is soft, porous, or showing signs of deterioration, apply Toupret Wood Hardener to stabilise the surface. Apply directly to the affected timber using the brush supplied in the screw top lid. Work it liberally into the wood, paying particular attention to end grain and any areas where the timber is soft or rotten. It can be applied to dry or damp wood, but not soggy wood. On sound timber, you can fill directly.
Step 2: Fill - Apply Flexible Filler
Before filling, make sure the wood is primed. Fill-Flex is a ready-mixed flexible filler that remains permanently flexible after setting. It is designed to accommodate movement within the substrate rather than resist it, making it the ideal choice for joints and junctions where timber naturally expands and contracts. Enhanced with microfibres, it offers excellent adhesion and bridging properties, helping to maintain a durable, long-lasting seal.
When using Fill-Flex out of a tub, use a filling knife; apply Fill-Flex firmly into the gap, pressing it in deep enough to cover both sides. Work along the joint in one direction for a clean, consistent fill.
Fill-Flex is also available in a cartridge (310ml). Secure into a caulking gun and apply along the crack or gap, ensuring full coverage.
Smooth the filler to a flush finish as you apply it. This is important - Toupret Fill-Flex is resistant to sanding once set, so the surface needs to be as smooth and level as possible during application. Use your filling knife, finger or damp sponge and smooth to your final finish.
For longer runs along skirting or architrave, work in manageable sections and keep a wet edge to avoid visible joins in the filler.
Step 3: Finish - Smooth for Decoration
Once Fill-Flex has fully set, apply Wood Filler as a finishing coat over the repair. It is available in two colours, Off-White and Natural Wood, choose the one that matches your timber or intended finish.
Apply a thin coat over the filled joint and feather it out onto the surrounding woodwork so there is no visible line between the repair and the original timber. This blends the repair and creates a consistent surface for decoration.
Allow to dry fully, then sand lightly with fine-grit sandpaper (P180 or higher). The repair is now ready for priming and painting. Because the Fill-Flex underneath remains flexible, the joint will continue to absorb movement through seasonal changes without cracking, giving you a lasting repair that stays sealed.