I sanded on a 45 degree angle and now the floor has rows of marks in it that won't come out by using same piece of sandpaper that I ahd used for a few passes. How can I remove marks?

Sand in the direction of the boards using a new paper. Stay with the same grit for the passes. If you ahve to resand the areas a second time in the same direction to get them out, then so me it. I think that if you change grits to a lower grit for this might leave sanding lines too deep for the next grits to get out. I like to stay with a higher grit and get them out with a couple passes instead of too much grit for the job. If you sand them out and there just a few visable, you can edge them out before using the next grit and passes. If after your sanding schedule and you see more lines, your going to have to resand. There isnt a quick fix. Good luck

45º is a little drastic. 20-30º is only for the extreme cases.

Like stated above, sand it with the grain now. It may take more than one pass with the 60 grit. Then move to your 80 grit, staying with the grain.

OK, will do.