If you have a soft spot in a window frame, a rotten sill corner or a gouge in an old door, the fix comes down to three materials people reach for: builders bog, a hardware store wood filler, or a two-part wood repair epoxy. They look similar in the tub. They behave very differently once they are in the timber and the weather gets to them.
Here is an honest look at what each one actually is, where it works, and where it lets you down. We repair timber windows for a living, so this is written from what survives on real houses, not from the back of a packet.
The 30-second answer
Use the right material for the job and you only do the repair once.
- Builders bog suits fast cosmetic fills on dry, stable timber that will be painted and kept out of standing water. It is cheap and sands beautifully.
- Hardware store wood filler suits small dents, nail holes and shallow surface repairs on interior joinery or protected timber.
- Two-part wood repair epoxy suits structural repairs on external timber that has to move, flex and shed water for years. This is what a window sill or a door bottom needs.
The rest of this article explains why.
What builders bog actually is
Builders bog is a polyester body filler, the same family of product used to fill dents in car panels. It is a paste and a hardener you mix on a board, and it goes off fast. Within about twenty minutes you can sand it, which is why panel beaters and quick renovators love it.
On timber, its strengths are speed and sandability. Its weakness is that polyester is rigid and it does not bond to wood the way it bonds to steel. Timber swells and shrinks with the seasons. A rigid filler sitting in a moving timber frame tends to lose its grip at the edges, and once water finds that hairline gap it sits behind the filler and the rot keeps going, out of sight. On an external sill that takes sun and rain, that is the common failure we see.
None of that makes builders bog useless. On a dry internal repair that will be painted and never sit in water, it does a tidy job. Just be clear about what you are asking it to do.
What hardware store fillers do well
The tubs of ready mixed wood filler on the shelf are built for small, shallow jobs. Nail holes, skirting dents, a chip out of a windowsill edge, a gap before painting. For that work they are quick, low fuss and easy to sand.
Where they struggle is depth and movement. Most are single-part fillers that dry by losing water or solvent, so a deep fill can shrink, skin over on top while staying soft underneath, or crack as it cures. They are also not designed to carry load. If you are filling a hole that the window still has to be strong around, a surface filler is the wrong tool. Our longer guide on what to consider before choosing a wood filler goes through the trade offs in more detail.
What a two-part wood repair epoxy does differently
A two-part repair epoxy is a resin and a hardener that cure by chemical reaction rather than by drying out. That single difference is why it behaves so differently in timber.
Because it cures by reaction, it does not shrink as it sets, so you can build a deep repair in one go without the middle staying soft. It soaks into and grips the surrounding timber fibres, so the repair moves with the wood instead of popping loose at the edge. Once cured it is hard enough to shape with a plane, drill, sand and paint, and it holds a screw. That is what lets you rebuild a missing sill corner or the bottom rail of a door and have it behave like solid timber again.
The trade off is that you work to a cure time, not to a twenty minute panel sand. It also costs more per tub than bog or shelf filler, which is fair, because it is doing a structural job on timber you would otherwise be replacing. If you want to see the method on a real repair, we wrote up a full rotted window sill repair step by step.
We go deeper into the head to head choice in wood filler or wood repair epoxy: which should you use if you are stuck between the two.
The products, compared honestly
We make two wood repair epoxies, and the honest difference between them is working time, not quality. Both cure by reaction, both take shaping and paint, both are made for external timber repair.
DT Wood Repair Epoxy 1 Hour Cure is the faster option. It suits smaller repairs, warmer days and jobs where you want to keep moving and get back to shaping sooner. On a hot day the working time comes back on you quickly, so mix what you can use.
DT Wood Repair Epoxy and Filler 4 Hour Cure gives you more open time on the board. That is the one to reach for on larger repairs, on cooler days, or when you want time to work the material into a corner and get the shape right before it starts to firm up.
If you are unsure, most homeowners doing a single sill or door find the longer working time of the 4 hour cure less stressful, because you are not racing the tub.
Frequently asked questions
Can I use builders bog on a window frame?
You can, on a dry internal repair that will be painted and stays out of the weather. On an external sill or a spot that gets rain, a rigid polyester filler tends to let go at the edges as the timber moves, so a two-part wood repair epoxy is the safer choice there.
Why does my old wood filler keep cracking and falling out?
Usually because it was asked to do too much. Single-part fillers are made for shallow cosmetic work. In a deep or moving repair they shrink and crack as they cure. A reaction-cured epoxy does not shrink and grips the surrounding timber, so it stays put.
Is wood repair epoxy stronger than the timber around it?
Cured repair epoxy is hard and holds a screw, and it bonds into the timber fibres so the repair works as one piece with the wood. The aim is not to be harder than the timber, it is to move with it and shed water, which is what keeps the repair sound over the years.
How do I choose between the 1 hour and 4 hour cure epoxy?
Choose by working time and conditions. The 1 hour cure suits smaller repairs and warmer days when you want to move fast. The 4 hour cure gives you more open time to shape larger repairs and is easier on cooler days.