Hi, Rob. Go back to *** ****'s Message Board and hit Control-F, then type in words like "hardwood" or "floor" to search the board. The answer is there several times, but the bottom line is that you don't fill the cracks. They are necessary for expansion and contraction. If you fill them, the cracks will get wider eventually and you will have more problems.
You might want to check the article about sanding floors in The Family Handyman magazine from a couple of months ago. It was very detailed and the pictures help a lot.