Rotten window repair

The kitchen window at the rear of the cottage is a single pane below a top opening casement window. The glass pane is held in place by a nailed wooden bead rather than putty glazing compound. The problem with this installation is that it is not water resistant. Over the past few years, rain has penetrated behind the wooden bead and has caused wet rot at the foot of the window. It needs the window replacing or the rot removed and repaired.

  

I've tried to interest local window companies in quoting for replacement wood frame window, but none seem interested in this relatively small job.  I could build a new window in my workshop and install it. However, I've never done this type of work before and as a "new window" it would need certification under building regulations because I'm not a registered FENSA installer. As the cottage is located in a Conservation Area, we don't want to have to tangle local planners by trying to install modern windows.

So I've decided to undertake a repair. I'll dig out the rotten wood, treat the remainder with wood treatment, then use Toupret wood hardener resin to provide a firm base. I'll then apply filler material to restore the shape and strength of the frame. I'll mix my own filler compound from epoxy resin, glass microbead filler powder and various fibres to use to fill the hole. Epoxy resin based filler has some flexibility similar to the original wood. It is also low odour, so it will not inconvenience our guests as it cures. Stainless steel wood screws in the interior of the hole will act as a key to help retain the filler in the hole. Once the filler is smooth and hard, I'll sand it down, and replace the wooden bead around the glazing with proper linseed oil putty. Then it will be time to repaint the whole window frame,

I've just ordered some fresh Gurit SP106 epoxy resin, which is now in transit to me. It's not cheap, but it will give a long lasting repair. I've used this technique on boats before to good effect.

Edit: 16th July 2019

I've finally had the time and weather to work on this project. I've dug out the dead wood from the window frame.

The damage was less extensive than I'd feared. I sprayed the bare wood with copper sulfate solution to kill off rot fungi and once that had dried I treated the soft wood with the Toupret Wood Hardener. This particular hardener is water based and serves to strengthen the wood fibre and also lock in the copper sulfate.  If the copper solution is not bonded to the wood it fairly quickly leaches away if the wood get wet. It leaves a greenish tinge to the wood.