Tung oil, soaks in about 1/4". Once it's on, no other finish or stain will 'take'. Stains can be sanded off tung oiled wood(Wood's wood. You finish stocks using the same techniques used on fine furniture), but not the tung oil.
Put on whatever stain you're using as per the directions on the can. Then apply a coat of tung oil, then rub it in with a lint free cloth. The bit of heat generated by the friction is a good thing. When it feels dry, leave it sit for 24 hours and repeat. A week or so of doing this will give you a nice sheen on the stock.
Not all stocks are walnut. Especially milsurp stocks. Tung oil will protect the stock, but it may not be as pretty as you'd like.
The biggest advantage of taking the time to put on an oil finish is that if the stock gets scratched, rubbing in a bit more oil makes the scratch go away. Even BLO.
Varnish can be removed with any commerical varnish remover.
Too much Tung
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