Best way to take polyurethane stain off wood or lighten it
by
Terry Keck
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Not sure if this helps.. https://www.bobvila.com/articles/bleaching-wood/
CitriStrip ought to be strong enough for any poly sealer. If the wood is stained, you will have to sand the wood back to the base. Since stain soaks into wood, you will lose a lot of surface getting to a stain. ☺️
http://homeguides.sfgate.com/strip-polyurethane-wood-99205.html