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i would heat the wood gently and keep wiping. or whatever is sticky may return

meths it, progress to acetone if you have to but try a spot on the inside of the foregrip first for safety.
i personally like birchwood casey stock oil myself, and then a coat of their stock wax...

depends on what the sticky is
was it finished with raw linseed, boiled linseed, swiss oil.......etc
i would start with detergent and water then progress to meths