Phil's Answer
Thanks for getting in touch. It’s hard to get an accurate idea of exact scale from your pics but from what I can see you can probably rescue that in the short term but ultimately that sill could do with replacing. The hole looks a bit deep for two part wood filler so what I would suggest using if you’re not going to replace the timber is a resin repair kit, something like ‘Repair Care Dry Flex 4’. You need to first apply a wood harder/ primer product that will sure up the edges you are filling to. The resin suppliers often do their own harder, otherwise you can use something like ‘Ronseal Wet Rot Wood Hardener’.
The resin repair kit is a two part system that you mould as it dries. Make sure you don’t use too much as its not something you want to be sanding back! If you can’t get it to mould to the exact shape you want you’re better off doing it slightly shy of where you want it to finish and then skim over the top with a two wood filler that can then be sanded. Ronseal High Performance Wood Filler is one example. You get a small tube of hardener with it. Approximate ratio is a golf ball of filler to a pea of harder (very technical!).
The area of the window itself you should be ok using a two part wood filler but again, I would recommend using a wood harder first on the surrounding area to make sure it’s sound. I recommend an 80 grit sand paper to sand the two part filler with as it’s slightly more coarse and wood filler can be stubborn to sand back. Once that’s done you can prime as necessary, undercoat and gloss. Make sure all the paints you use and suitable for exterior work. This repair won’t last for ever but if done correctly should give you a few more years before you have to replace the timber. Hope that’s helpful.
