What is wood rot and how does it form?

Wood rot is fungal decay that develops when wood stays damp for long periods. It often starts where water can sit or get trapped. Like window corners, sills, joints, horizontal ledges and end grain. If paint or sealant is damaged, moisture can penetrate deeper and the rot can spread from the inside out.

• Moisture exposure (rain, condensation, leaks)
• Slow drying (shaded or “cold” sides of a building)
• Damaged paint lets water into the wood
• Fungi break down wood fibres over time

How to spot wood rot (symptoms)

• Discoloration (dark brown, black, rusty tones)
• Soft wood (a screwdriver sinks in easily)
• Peeling or bubbling paint
• Hollow sound when tapped

Helpful tools (Batavia)

With the right tools you work cleaner, faster and more precisely, especially in tight window corners and edges.

• 18V Cordless Oscillating Multitool – cut out rotten wood accurately and refine edges
• 300W Multitool – Incl. Anti-Vibration System – controlled cutting with less vibration
• 18V Drill Driver – pilot holes, removing trim screws, opening joints if needed
• 12V Cordless Detail Sander – smooth filler and prep for painting
• Workbenches – stable working surface for safer repairs

Step-by-step: remove and repair wood rot

Step 1: Remove all rotten wood

Remove every soft, damaged part until you reach solid, dry wood. Also cut back a small margin of healthy wood around the rot to ensure nothing remains that could continue spreading.

For precise removal in corners and tight spots, a multitool like the 18V Cordless Oscillating Multitool is ideal.

Step 2: Brush out debris and let it dry

Brush away loose fibres and dust, then let the area dry thoroughly. Depending on weather and exposure, drying can take 24–48 hours (or longer in cold, damp conditions).

Step 3: Apply wood hardener / rot treatment

Apply wood hardener to the exposed wood. This strengthens weakened fibres and helps the filler bond securely. Allow it to cure as instructed by the product.

Step 4: Apply wood filler and rebuild the shape

Mix your wood repair filler according to instructions (many are 2-component) and press it firmly into cavities. Overfill slightly so you can sand back to a flush finish.

• Mix only what you can use before it starts curing
• For deep holes, build up in multiple layers
• Damaged corner? Use a simple “form” (two small boards) to recreate a sharp edge

Step 5: Sand, prime and repaint

Once the filler is fully cured, sand the repair smooth. Remove dust, apply exterior primer and finish with quality exterior paint to seal the repair from moisture.

For clean transitions and smooth edges, the 12V Cordless Detail Sander is perfect.

When should you repair vs replace?

Repair is ideal when rot is limited to a small area (for example, a localized corner or section of sill). If the damage is extensive, the wood is soft across a large area, or structural integrity is compromised, replacing a section may be the safer choice.

• Repair when damage is local and surrounding wood is solid
• Replace when a large portion is affected or the frame feels unstable

How to prevent wood rot (maintenance)

• Keep paintwork and sealant in good condition (fix cracks early)
• Inspect window frames, sills and corners at least once a year
• Make sure water can drain away (no “water traps” on ledges)
• Fix leaking gutters and joints quickly
• Promote drying and ventilation where possible

Ready to fix wood rot properly?

With careful prep and precise removal, you can repair wood rot in window frames and exterior woodwork for a clean, durable result. Use a multitool to cut out rot accurately, then sand and repaint to protect the repair for the long term.