Filling a split or crack in a timber window frame is well within reach of a confident DIYer, provided the damage is minor to moderate. With the right preparation, a quality filler, and a good paint finish, you can restore the frame's appearance and — crucially — keep water out before it causes serious rot.
Assess the Damage First
Before reaching for the filler, take a good look at what you're dealing with. Run your finger along the crack and probe it gently with a screwdriver or awl.
- Hairline or surface cracks – usually cosmetic, caused by seasonal timber movement or paint failure.
- Open splits – wider gaps, often from drying or impact. These need a flexible or two-part filler.
- Soft, spongy timber around the crack – a warning sign of rot. If the wood feels soft or crumbles, you may have a more serious problem (see below).
- Structural damage or large sections missing – this goes beyond simple filling and may need a joiner.
Also check whether the frame is painted or bare timber, as this affects your product choice.
Tools and Materials You'll Need
- Stiff brush or vacuum (to clean out the crack)
- Screwdriver or awl (to probe and clear loose material)
- Sandpaper (80–120 grit for prep, 180–240 grit for finishing)
- Putty knife or filling knife
- Timber filler or two-part epoxy wood filler
- Primer and exterior paint
- Masking tape (optional, for neat edges)
Choosing the Right Filler
Not all fillers are equal. For exterior timber window frames exposed to the NZ climate — which can swing from hot, dry summers to wet, cold winters — flexibility and adhesion matter a lot.
| Filler Type | Best For | Notes |
|---|---|---|
| Acrylic timber filler | Small cracks, painted frames | Paintable, easy to sand, some flexibility |
| Two-part epoxy filler | Larger splits, structural voids | Very strong, low shrinkage, fully paintable |
| Linseed oil putty | Glazing rebates, traditional joinery | Traditional option, slow drying |
| Flexible gap filler / sealant | Moving joints, junctions | Use where timber meets other materials |
For most exterior cracks, a two-part epoxy wood filler (available at Bunnings or Mitre 10) gives the most durable result. Brands like Selleys and Bostik make widely available options. For smaller surface cracks on painted frames, a good acrylic timber filler works well.
Step-by-Step Repair Process
1. Clean and Prepare the Crack
Remove any loose paint, debris, and dust from the crack using a stiff brush or vacuum. For wider splits, use a screwdriver to clear out any loose fibres. The filler needs to bond to clean, sound timber.
2. Sand Back the Surrounding Area
Lightly sand around the crack with 80–120 grit sandpaper to give the filler a good key. Wipe away dust with a damp cloth and allow to dry fully.
3. Apply a Primer (if using epoxy filler)
Many two-part epoxy systems include a liquid consolidant or primer — apply this first and let it soak in. This strengthens any slightly soft timber and improves adhesion.
4. Mix and Apply the Filler
For two-part fillers, mix according to the manufacturer's instructions — work quickly as pot life can be short (often 5–15 minutes). Press the filler firmly into the crack with a putty knife, slightly overfilling to allow for sanding back. For deep cracks, apply in layers if the product requires it.
5. Allow to Cure, Then Sand Smooth
Once fully cured (check the product datasheet for times), sand back flush with 120 grit, then finish with 180–240 grit for a smooth surface ready for painting.
6. Prime and Paint
This step is critical for exterior frames. Apply a quality exterior primer followed by at least two coats of exterior paint. Resene and Dulux both offer exterior timber primer and topcoat systems suitable for NZ conditions. A good paint film is your best defence against moisture re-entering the repair.
Dealing with Rot
If you find soft, punky timber around the crack, don't just fill over it. You'll need to cut back to sound timber or use an epoxy consolidant and filler system designed for rotted wood. For significant rot, especially near the sill or bottom rail where water pools, it's worth getting a joiner to assess whether the section needs to be cut out and replaced. Painting over rot never ends well.
Weathertightness and the NZ Building Code
Timber window frames are part of your home's weathertight envelope. Under the NZ Building Code (particularly Clause E2 – External Moisture), external joinery must prevent water ingress. A cracked, unpainted frame can allow moisture in, leading to rot, mould, and potential Healthy Homes compliance issues for rental properties. Keeping frames well maintained and painted is one of the simplest ways to protect your home. BRANZ has good guidance on maintaining timber joinery.
When to Call a Professional
Call a qualified joiner if:
- The crack is large, structural, or the frame is badly rotted
- The window no longer opens, closes, or seals properly
- You suspect the frame has moved or the building has settled
- The damage affects the glazing rebate or the window's weathertightness
- You're unsure whether a building consent is required for any replacement work (always check with your local council — some joinery replacements may be covered under Schedule 1 exemptions, but confirm this with MBIE Building Performance)
A skilled joiner can splice in new timber, replace sections, or advise on full frame replacement when needed. You can find qualified local joiners on tradedirectory.co.nz.
