When it comes to DIY window repair, one common question is what type of filler to use for window frames. The answer depends on which part of the frame needs filling and the type of finish you plan to apply after repairing it. Keep in mind that some wood fillers are only suitable for indoor use, while others are formulated for outdoor use.

If you plan to paint the window frame, you can use a filler with a solid opaque color that will be covered by the paint. However, if you plan to use a semi-translucent coating system like Sikkens or Sadolin, it’s best to use Fiddes Wood Filler Gel. This type of filler is mixed with sanding dust, preferably from the same type of wood being filled, to create a paste that is half made of wood. This allows it to absorb the finishing product in a similar way to the surrounding wood, resulting in a more seamless repair.

Filling should follow the splicing in of new timber

It’s important to note that filling should follow the splicing in of new timber. If the new pieces have been cut well, there won’t be much filling required. Large volumes of filler should be avoided due to the nature of wood expanding and contracting with changes in moisture content. The filler doesn’t expand the same way as the wood, which can lead to cracks appearing. Therefore, it’s best to keep filling to a minimum and bulk out any repairs with timber. Only use filler for small holes or gaps, such as the holes of screw heads, small gaps around spliced-in edges, and thin surface filling.

It’s essential to understand that wood moves, but filler doesn’t. This means that the amount of filler used should be carefully considered. Additionally, the moisture content of the frame and new pieces will be different. The frame being rotten indicates that the water content is high and will be swollen, whereas an old piece of timber stored dry for splices will not shrink back but may expand. Over time, the frame and new piece will stabilize. The old frame sections are likely to dry out and shrink as the repairs have stopped moisture getting in to the wood. The flexibility of the filler may not be enough to compensate, so gaps and cracks can be expected. Therefore, final finishing and painting should be left for weeks (depending on the weather) so that any remedial filling can be done.

Epoxy wood filler

As for the type of filler to use, I personally recommend using epoxy wood filler. It’s important to mix it in small batches and press it in well. After half an hour, it can be sanded or pared back with a chisel, and any low spots can be filled again. It’s not worth doing a big fill all at once as you’ll end up with a lot of paring back and sanding to do.

While you may be tempted to aim for a perfect, invisible repair, keep in mind that major time spent on perfection at this stage may be undone as the timber stabilizes. Instead, focus on a careful and thorough repair that will keep your window frames in good shape for years to come.

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