Set any protruding nails at this point and do any repairs of split boards. But don't fill all the nail holes at this point. If you apply a stain on numerous filled holes, these will turn quite dark, almost black. This may give the floor a peppered look if there are too many. Fill the holes after the stain and at least one coat of finish is on the floor. You will then be able to match the filler to the color of the floor. And you may find that you have to use different colors in early and late wood, like oak, to get a good match.
I always start off sanding a new floor and most old floors with 36-grit sandpaper. Use high drum tension but be careful not so high a tension that the machine starts to vibrate or stall. This alone can cause chatter marks. Be sure to use the silicone carbide (SiC) or for really hard finishes (or really dense wood like hickory) try the new zirconia-alo variety. Both have their economical uses. I'll be doing a separate article on sandpaper types, as this is a complicated subject itself.
On old floor I want to remove ALL the old finish, down to the clean white wood, no exceptions. Keep sanding until the wood is clean, or your fine sanding will never work. Fine sandpaper clogs too easily on still dirty and waxy floors. On new floors I want to remove all the over wood efficiently and quickly, so this coarse 36-grit paper does it's work quite well. I will pencil mark any uneven board ends, so that when I go over it again I can easily see if I have sanded these flush. This is especially important in older houses with very uneven subfloors. I commonly have to sand the newly laid hardwood floor at a 30-degree angle at first to help level the new wood. I will always do this on a newly installed parquet floor. If you don't do this, your machine may skip over the low boards. If you don't see this before you stain the floor, this will create a defect in your stain, as it will be apparent that these boards didn't get sanded. You may have to start the job all over if the customer complains.
Oh, and I might add that when resanding those prefinished floors it might be best to use the zircon-alo sandpaper. You have to accomplish two things with the coarse sandpaper, one is to level the prefinished floor for the first time. The installers of these varieties of floor rarely do any prep of the subfloor, so it will be really critical that these floors be sanded at a 30-degree angle to flatten out the entire floor surface. And you should try to remove those annoying side and end bevels.
On the latest generation of prefinished wood flooring these should be quite shallow and you should be able to remove the beveled edges but not take too much wood off. But on the last job I was on, I had to scrape the V grooves out by hand after the coarse sanding. The V groove was just too deep, and I would have removed far too much of the wood, if I had tried to sand them out. And these hard factory conversion finishes really impede the progress of the coarse sanding. The zircon-alo 36-grit paper becomes worth its extra expense in this situation. You won't have to move to coarser sandpaper using the zirc/alo to get through these finishes. If you sand with say a 24-grit SiC paper you will have to do an extra sanding with 40 or 36-grit to take these deep 24-grit trenches out. This turns out to be a real waste of time and sandpaper.
If I have to sand a floor at an angle, I later straighten out the sanding lines with the same somewhat worn 36-grit sandpaper still on the machine. But if the floor is more than 300 square feet I will change to a fresh sheet of 36-grit, and just dull it a little with a piece of fine sandpaper pressed to the drum as it is running on the machine. You can make higher-grit sandpaper with this burnishing method but be careful to do it evenly across the whole sheet spinning on the machine.
As it wears down the broken edges of the silicone carbide remain somewhat sharp and effective. After the coarse sanding, I then get on my hands and knees and take a good look at the sanding lines and make sure all the angled sanding lines and high spots are removed. If not I will again pencil mark any defects and sand those spots again with the same 36-grit sandpaper. Don't ever expect the medium-grit sandpaper (60-grit) to take out any of these defects, it simply doesn't have the power.
Step 1: How To Sand Wood Floors: Without Leaving Machine Marks Introduction
Step 2: How To Sand Wood Floors: Can it be Sanded?
Step 3: How To Sand Wood Floors: Start with Fine Tuning Your Drum Sander and Choosing the Best Sand Paper
Step 4: How To Sand Wood Floors: The Right Sand Paper Makes The Job For The Initial Rough Sanding 36 Grit
Step 5: How To Sand Wood Floors: Repairing Gaps and Second Sanding Stage With 60 Grit
Step 6: How To Sand Wood Floors: Final Drum Passes with Burnished 80 Grit or Regular 100 or 120 Grit
Step 7: How To Sand Wood Flooring: Edging - Using The Edger Without Effort
Step 8: How To Sand Wood Flooring: Screening and Final Preparation For Staining or Finishing