If you have patched a timber window with builders bog and you are wondering how long it will hold, the honest answer is that it depends on where it is. Indoors, out of the weather, a bog repair can sit there for years. On the outside of a window that faces the sun and the rain, it often starts to crack and lift far sooner than most people expect, sometimes within a season or two.
Here is why the location matters so much, and what actually happens to bog once it is on a window that moves.
The short answer
Builders bog does not have a fixed lifespan. How long it lasts comes down to three things:
- Where it is. A protected interior reveal is a very different job to a weather-exposed bottom rail or sill.
- How much the timber moves. Timber that swells and shrinks with the seasons puts constant stress on a rigid filler.
- How well it was sealed. Bog that is primed and painted properly, and kept painted, lasts longer than bare or poorly sealed bog.
On an exterior timber window in the Australian climate, with full sun, driving rain and timber that never stops moving, bog is usually a short-term fix rather than a lasting repair. That is not a knock on the product. It is doing a job it was never designed for.
What builders bog actually is
Builders bog is a polyester body filler. It was developed for panel beating, filling dents and dings in steel car bodies. On steel it is excellent, because steel is stiff, it does not swell with moisture, and the filler is sealed under automotive paint straight away.
Timber behaves nothing like steel. A timber window takes on moisture when the air is damp and gives it back up when the air is dry, so the wood is always moving a little. A filler built for a rigid steel panel is being asked to live on a surface that will not hold still. That mismatch is the root of almost every bog failure on a window.
Why it fails on a timber window
Once builders bog cures, it sets hard and rigid. It has very little give. The timber around it keeps moving with the seasons, and because the bog cannot move with it, something has to give. Usually it is the bond at the edge of the repair.
The typical failure goes like this:
- A hairline crack opens around the edge of the bog patch, where the filler meets the timber.
- Rain and dew work into that crack and sit against the bare timber behind the filler.
- The trapped moisture has nowhere to go, so the timber stays wet and starts to rot behind what looks like a sound repair.
- The patch eventually lifts or pops out, often taking a flake of paint and a lump of softened timber with it.
The frustrating part is that the surface can look perfectly fine right up until it fails. The paint hides the crack, and the first sign of trouble is often a patch that sounds hollow when you tap it, or a corner you can lift with a fingernail. We go into this in more detail in our piece on why you should beware of a patch and paint job.
Indoors versus weather exposed
Indoors, builders bog can last a long time. An interior window reveal or a sash that stays dry is not getting rained on, the temperature swings are gentler, and the timber moves less. A small cosmetic fill in that spot might never give you any trouble.
Outdoors is where it struggles. The parts of a sash window that cop the most weather are exactly the parts people tend to bog: the bottom rail of the lower sash, the sill, and the bottom corners where water likes to sit. These are high movement, high moisture spots, and they are the worst place to ask a rigid filler to hold on.
So if someone tells you their bog repair has lasted ten years, they may well be right, and it may well be on a windowsill indoors or somewhere sheltered. The same filler on an exposed external rail is a different story.
What lasts instead
The fix that actually holds on a moving timber window is a two-part wood repair epoxy. The difference is in how it behaves once it cures.
A proper two-part epoxy soaks into and bonds with the timber rather than just sitting on top of it, and it keeps enough flexibility to move with the wood instead of cracking away from it. Because it bonds into the fibres and shares the timber's movement, water cannot creep in behind it the way it does behind a rigid bog patch. That is the whole reason it outlasts body filler outdoors.
On timber that has gone soft, it is also worth stabilising the surrounding wood first. An epoxy wood stabiliser soaks into the punky fibres and firms them up so the repair has something solid to bond to, which stops the rot carrying on underneath. For a general rot repair the steps are set out in our guide to fixing wood rot in a timber window.
If you are weighing up body filler against epoxy before you start, our comparison of builders bog versus epoxy and the broader question of wood filler or wood repair epoxy both go through where each one earns its place.
For most people doing a window repair, the simplest way in is the DT Epoxy Starter Pack, which brings the stabiliser and the repair epoxy together so you are not buying parts piecemeal.
Frequently asked questions
Does builders bog crack on timber?
Yes, this is the most common way it fails on a window. It cures hard and rigid, and the timber underneath keeps moving with changes in moisture, so a crack opens around the edge of the patch. Once that happens water can get in behind the filler.
Can you use builders bog on window frames?
You can, and indoors or in a sheltered spot it may hold for a long time. On weather-exposed parts of a window, like the bottom rail or sill, it tends to crack and lift because those areas move the most and cop the most moisture. For those spots a two-part wood repair epoxy is a far more durable choice.
How long does builders bog last outside?
There is no set figure, and anyone who quotes you an exact number is guessing. Outdoors on a moving timber window it often starts to crack within a season or two, though a well sealed and well maintained patch in a sheltered position can last longer. The point is that it is a short-term fix in the weather, not a permanent one.
What is better than builders bog for a timber window?
A two-part wood repair epoxy. It bonds into the timber and moves with it instead of setting rigid and cracking away, so it keeps water out and lasts far longer on exposed window timber.