My hardwood floors are 11 years old. The kitchen floor has some grey areas and grey streaks. Based in an internet search I know that the grey spots and streaks means that I've worn through the polyurethane and the wood has oxidized. The internet sites I have visited say that to repair I must sand the entire kitchen floor down to bare wood and re-polyurethane. How does sanding the floor down to bare wood get rid of the grey areas in the wood? Do I keep sanding and removing wood until I get past the grey? If so, doesn't that mean I now have a low spot in the floor where sanded the wood to remove the grey? Thanks for your help.

When finish is worn off to bare wood there is certainly a low area. Sanding a little more to get the stained area back to bare wood is the easiest part of your refinish.
Sanding the old poly-urethane around the freshly exposed wood will start to make it even. You probably will never notice any difference, once the old poly is gone.

Thanks, Pete. So the grey stain in the wood is very thin and easily sanded away? By the time I have sanded the entire kitchen floor to bare wood would you expect the grey areas to be gone? Or should I expect to spend more time sanding the grey areas than I spend sanding the rest of the floor?