I have a rental home with a real wood floor, we sanded the floor and used several coats of urethane when I bought the home, the last renters had a dog that apparently urinated in several spots, how did the urine penetrate the urethane? and is there a way to remove the stains with out having to re do the entire floor? and would more coats of urethane stop this from happening again?

 

I assume the stains would need to be sanded out but don't know for sure
How much of the floor needs to be redone depends on where the stains are located and the layout of the room. The poly protects the surface but if there are any cracks between the boards, liquid can seep in there and then get absorbed by the wood under the finish.

 

Yeah, sanding and maybe even bleaching are going to be required, which means restaining and refinishing afterword.

We put ceramic in our units, would never consider wood for maintenance reasons. You have wood, not suggesting removing it, just throwing that out to you for future consideration.