I have problems aright. We put down red rosin paper for floor protection and it got wet and bled through a newly refinished floor. Is there any trick to removing this stain? If sanding is the only option can I sand only the boards that are affected? I know that would be tricky but I think I could do it. Has anyone tried that? Thank you.

you should be able to get away with scuffing the poly on the affected boards. IF you are doing it by hand you could use 180 or 220 grit abrasive paper .If you are using a disc buffer, I wouldn't recommend going below a 220 grit screen. 3m maroon pads also work well in this case.

Once you clean the leakage out of the poly , you can re coat using the same coating that was used previously .