Stock refinish project - few questions

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Morning all - I scored a stock for my JW103 off traders recently and have embarked on a rebuild project.
So far have stripped it right back, paint stripper did next to nothing so used a scouring pad on the drill - now in the process of reshaping and sanding back.
I've pinned the grip in a couple of places to add some strength.

Couple of questions though - I want to put poly over the top of a darker stain, as the wood is pretty soft and I want a harder finish - should I be putting oil poly over oil stain?
And secondly, I'd like to extend the grip slightly and put a grip cap on - anyone got any bright ideas about what to use for a grip cap?

Hiya. Yep oil poly over any oil stain. Re the grip extension, the worlds your oyster.. perhaps a piece of scrap native timber, shaped to suit?