What is the maximum depth that you can safely sand a standard ¾ inch thick hardwood floor before affecting its strength and structural integrity?
Generally you can sand a hardwood floor until the top layer of the groove is gone, which is about ¼ inch. However, you don’t always have to do an aggressive sanding job to get the floor ready to be refinished.
If your floor finish is simply worn and has no major physical damage (like deep scratches, gouges, etc.) then I would not recommend using a floor belt sander. This is the kind of machine that has a 12 inch wide rotating sanding belt. In untrained hands, this machine is also fastest way to permanently damage a floor and is therefore best used only by floor pros.
There are two better options for the do-it-yourselfer. First, you can rent a floor buffer with a sanding screen. This takes off the uppermost layers of the old finish and gets the floor ready for a new coat. It is easy and inexpensive.
For floors requiring a bit more sanding, I’d recommend renting a machine called a U-Sand. This machine has 4 rotating 6 inch sanding disks under one hood that serves to vacuum the dust off the floor as it is created. It will not damage your floor and is also easy to use.
After sanding, clean the floor thoroughly and then use a lambs wool applicator to apply a good quality oil-based polyurethane. Do not use water-based polyurethane on the floor. It is not nearly as durable as oil-based and with all the work that goes into refinishing a floor, you don’t want to be doing it again any time soon!