I went to Home Depot to rent a Square Buff Floor Sander to refinish my Kitchen floor. They said I could use 20, 60, 100 grit sandpapers to do the job. Please note that they do not have any sandpapers of grits of 30, 36, or 40.
The floor is 35 years old and has never been sanded. It has several layers of clear coatings on it.
I wonder if it is too aggressive to start with grit 20 and then go to grit 60.
I've only refinished two hardwood floors myself but I do know a bit about woodworking given my dozen years of experience as a cabinet maker. The first floor was done with a rental rotary sander. This violates one of the cardinal rules of wood sanding: always sand with the grain. A rotary sander will leave swirl marks and you'll have to spend a lot of time with increasingly finer grits to remove all of the swirl marks. I started with 36 grit which was necessary due to some staining of the oak and progressed through 120 grit in roughly 20 grit increments. This gets expensive fast! The second floor was done with a belt sander like the pros use. You sand with the grain so overall you have less sanding to do since you don't have to remove swirl marks. There are two major caveats with a drum/belt sander. First, the ones from rental places are usually way out of adjustment. Which leads to the second caveat, it really helps to know what you are doing, both for the adjusting and the sanding. Otherwise you end up with lap marks where there are seams between the adjacent passes. Another issue is ending up with a washboard effect. Tread cautiously! There might be some YouTube vids on how to use a drum/belt sander.
To answer your question directly, don't use anything courser that 36 grit and in fact given how good this floor looks you could probably start with 50 or 60 grit. Good luck!
I'd only consider 20 if you know you need to remove significant material--such as when dealing with deep scratches or stains. Otherwise I'd start with 60. If that doesn't work quickly enough on the varnish, move to 36 or 40 (from elsewhere, if necessary).
The coarser you go the more you have to work to remove wood in the later stages.
I'm not a pro. I've done one bedroom refinish myself and watched a number of large floors being installed when I was a builder.
Try starting with the 100 grit and see what happens. You can always switch down to a coarser grit and continue, then return to the finer ones later.
From your photos, looks like the wood itself is in decent shape and you're only needing to add some tooth, before adding another coat.
Start with the fine stuff, if it's too slow go for the coarser stuff.
From the look in the photo, 60 or 100 would probably be suitable for the first pass.
Hire a drum sander for the middle of the floor and an edging sander to do the perimeter. Because the edging sander works across the grain you'll need to use a finer grit on that one - perhaps 240.
If you want to take less than 1/16" thickness off the floor, 20 grit is definitely too coarse.
As one of the comments asks, what's wrong with the floor? It looks pretty good. If you're just wanting to add a new coat of finish, use a fine grit (120 or even higher) to scuff up the floor before laying down a new finish, otherwise it won't adhere properly.You might even be able to get away with a circular floor buffer with a scotchbrite pad (probably green or gray).
If you really need to sand the finish off because there are damaged areas we're not seeing in the photo, proceed carefully. The square buff floor sander from Home Depot (or other sources) will almost certain prove unsatisfactory for anything other than just the scuffing suggested above. You will need to use a professional class drum or belt sander. Think twice before doing this yourself. There are very hard to control. Even pros leave the occasional ripple in the floor and many first-time users leave the floor worse than when they started.
I would suggest trying to find a U-Sand floor sander. These sanders contain four 6" random orbit heads, making them far easier to control. They're as heavy as a pro drum/belt sander and having honking big motors, so they cut reasonably aggressively. Because they have flat contact with the floor, you are far less likely to leave ripples or ridges (though I'm sure it can be done). The U-Sand sanders use standard 6" hook and loop disks, which will probably prove to be more economical than the oddball size drums/belts you'll have to buy from the rental place if you use a drum/belt sander. The USand website has a search page for rental centers that carry their products. Don't be surprised if you can't find one close by, though.
Make sure you have a clear understanding of the rental shop's policy on unused sandpaper. You don't want to have to drive back to the store halfway through the job to get more sandpaper, but you don't want to be stuck with unused sandpaper, on the other hand. In my not-very-recent experience the rental shops I used gave 100% credit on unused abrasives, but make sure this is clearly stated on the rental contract.
And just to reiterate what multiple answers have already said - do not start with 20 grit unless you are trying to remove heavy damage. Even 36 grit is aggressive unless there are deep scratches and dents you are trying to remove. If the photo you provided is any indication of the condition, I'd suggest starting at 60 or even 80 grit. If that's not aggressive enough, step back down. Starting with too fine a grit wastes a little time, but does no damage. Starting with too coarse of a grit will cost you LOTS of time to correct and may cause damage you won't be able to sand out.
It's difficult to say. I'd try the 20 on a small area and see how it works. It's often a fine line between too aggressive and too fine where too aggressive removes too much material and too fine clogs up too quickly with the floor finish.
You may have to experiment some to determine what works best for your situation.
As others have hinted or stated outright, sanding can make a floor look worse. I moved into a house where someone had done a poor job of sanding the living room floor and left it with a ripple effect. The floor you show looks perfectly fine to me. The somewhat used look has an antique effect that many people find charming and desirable. Unless there are lots of marks and gouges not shown in the photo, I'd personally leave it alone. If you are determined to proceed, the advice from Criggie to start with the fine grit and change to the coarser grit if necessary is very sensible.
Your question doesn't indicate if you have any experience in doing this sort of work. If you don't, then you shouldn't do it without practice. I know that wood is expensive these days, but before beginning this job you should get a sheet of plywood and practice. Then move up to a few square feet of tongue-and-groove flooring and practice on that.
