Not glass fibre, though you
could reinforce with glass fibre or some such I guess. It's a two-pack epoxy resin, presumably somewhat akin to araldite, though sold as a wood filler. Quite expensive, as two tubes (mastic sealant sized) plus a couple of pots of the more liquid two part "primer" was around £80.
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I've previously used "Abatron" brand from Axminster but now out of stock, so went for this stuff.
Abatron is more of a putty which you mould in your (rubber gloved) hands but less sticky. I think this new stuff (3c) is easier to apply with a putty knife than Abatron.
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Again there's a two pack liquid primer layer. The primer slightly soaks into the wood and gives a surface the filler will stick to
Both are machine-sandable and can be shaved with a wood chisel once hard.
Despite the cost, I'll be using this kind of stuff from now on. I had very good results on my three floor bay windows a few years back, and it's totally held up.
Also got this two part stuff which is more for masonry and has something like stone dust in it.
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