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personally, I would strive to remove all the old polyurethane finish. This will take a while. The reason for this is, I have found that new poly really will not anchor properly to old poly.
therefore, I would sand all the poly off. Then I would rough up the underlying wood and sand in the direction of the grain only. this would be to just get a nice clean surface.
Then I would apply my poly, either oil or water based. This depends on you. There are many pro's and con's concerning oil verse water.
I will not get into these as they sometimes cause some people to get upset.
But, you check them out or email me personally and I will get you up to speed on them.
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