No advice about the sanding, but just why you want the seamless look is beyond me. If that is what you want, then any auto body filler is the wrong thing to use--wood moves with the weather, and body filler will just crack and pop out. Stick with a product made for wood--or better yet--if you are sanding, mix the sawdust with some glue and make your own--it will match the wood already there.

No advice about the sanding, but just why you want the seamless look is beyond me. If that is what you want, then any auto body filler is the wrong thing to use--wood moves with the weather, and body filler will just crack and pop out. Stick with a product made for wood--or better yet--if you are sanding, mix the sawdust with some glue and make your own--it will match the wood already there.

Seamless wood floor??? Whatever for. It would be cheaper in the long run just to put a new floor down ,

concrete maybe? (:)

try 100% orange oil on the carpet glue. If you're lucky it will soften and scrape right off. We used it to get tile adhesive off our basement floor; worked better than toxic strippers in a side by side test.

Just use same precautions you would use for mineral spirits to avoid spontaneous combustion.

Be advised that the four head sander (from U-sand) does not rotate; it's a random orbit. The heads move less than 1/4" as they vibrate. I never used a 3-head, but I would be surprised if any of these moved full-circle rotation.
I have used these numerous times, and I have found that they are very slow to remove the awful scratches cut by the coarsest papers (36 grit).
IMO you are better off to: employ a pro; rent a drum sander; or scrape off the glue by hand, and then use the U-sand with reasonable grits (100).
Casey

"What is a good trowel filler for the gaps when covering the ENTIRE floor?"

strip type hardwood floors expand ad contract throughout the year as the moisture content of the wood varies.

Typically lower in the heating season and higher moisture in the cooling season.
No finish can stop the movement, ad filing in gaps between strips will result in buckling (if done during the heating season) when the wood then tries to expand, or the gaps opening anyway if filled during the cooling season when the floor shrinks during heating season.

If you want a 'seamless' floor hardwood is not for you.

FWIW, I've had old carpet glue come off with hot water & bit of scrubbing. (Crazy, right?). I started out using Murphy's (gross, smells like bugs, to me!), then realized the hot water "rinse" & nylon scrub brush was just as effective without the soap. I know not all adhesives are/were water soluble, I only mention it because I'd never have thought to try it - especially if I'd planned to use a sander, anyway. I'd have gone straight to gumming-up sandpaper, also. :-)
I've got to ask - where did you hear good things about Bondo on wood floors? Do you know someone that's done such a thing? I also can't imagine it lasting very long - as said above, wood floors will move. Bondo will not.

That said, I noticed a key word in your OP, "rather" seamless. Are you talking about filling in some larger gaps, not a "gym floor"? Sounds like you're putting a lot of time, work, & money into it - why risk all of that & not being happy with the final results? Whatever you use - make sure it's made for use on wood (not a pickup truck, lol).

Good luck with it - can you post a pic of the floor?

Gym floors use very narrow strips for a reason.

Your wood floor WILL move seasonally, and no matter how or with what you fill the gaps, it will continue to crack and buckle. Unless you plan to paint the floor, Bondo in the floor cracks will stand out like a Zebra's stripes, in addition to cracking and coming out. Better to refinish the floor as is, and live with the cracks, or replace the floor. If you use an engineered floor, it will not move noticably, but will still be hardwood of your choice on the top layer.

"If you use an engineered floor, it will not move noticably, but will still be hardwood of your choice on the top layer."

If you use an engineered floor you will have to tolerate the clearance at all the edges since engineered moves in all directions (strip floors only move across the width of the strips).

Engineered wood floors move as a single large piece (AKA 'floating').

It is also a rather large number of the room has larger dimensions (3/4 inch+).
It can be hidden with full thickness baseboards and quarter round (or shoe mold).

I'm just glad you're not using carpet glue to refinish the floors. Guess I misread that topic!

Thanks for the help guys. I'm just going to use a drum sander and edger instead since I think the drum will hopefully cut through the glue better without clogging up the paper. I heard from a few flooring guys that Bondo has worked perfectly fine for them and accepted stain as well, but I just picked up some of the Varathane Red Oak stainable filler and I'm just going to patch up the larger and more noticeable gaps and it should be fine then.

I did replace 1 board and have to cover up some old registers with new boards. Originally, I thought it was pretty cut and dry and the floor was 1.5" x 3/4" Common Red Oak of some sort, but after viewing some images of #1 and #2 Common Red Oak samples, they don't seem to match up quite as well. The floor I am working with isn't as "knotty" and has a much larger variation in terms of grain patterns and color. Any idea what kind of wood and grade this is and where I could get 1.5" strips of it at? Worst case scenario, I could always just miter down 2-1/4" boards at Menards on the groove side and regroove them.
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"Bondo has worked perfectly fine for them and accepted stain as well, but I just picked up some of the Varathane Red Oak stainable filler"

Keep in mind that even stainable filler does NOT absorb and color like real wood.

"Stainable" just means stain can stick, not what color you may end up with.

That's true brick, but you can add the sawdust into the filler and it stains about the same since you are adding the same wood you are staining into the filler. Sorry, I left that part out.

I'm sorry - I don't want to sound rude at all, but the flooring shown - with the exception of missing register areas - looks terrific to me!

Are the register-holes the reason you're refinishing all the flooring? It looks great to me... I see warm toned, glossy finish. Smooth. No cupping of individual boards. No cracks worth filling. Maybe post a pic of the ugly floor, lol?

I'm no expert, the others are. But if my flooring looked that good - I'd pop the register covers back on & enjoy it!

"you can add the sawdust into the filler and it stains about the same since you are adding the same wood you are staining into the filler. "

Sawdust and hide glue are about the only ting that comes even close to the same color, and even then mostly with aniline dye, not pigment stains.

I still (occasionally) use shellac sticks, colored markers, artists oil paints (the stuff in tubes), and some other tricks to make matching repairs.
It is a painfully slow process (and that means expensive) to create a color and figure that match an existing finish on a repair.
One of the chief 'tricks' is to achieve most of the color in the filler material while making SURE it ends up lighter in color than the lightest color needed for the repair.
All you then need to do is add the darker portions to make the repair blend in.

I have done a couple repairs where the owner could not even find the repair at first and still thought the bill was "very reasonable,"

It is nice to have both a happy customer AND get well paid.

I would NOT try a drum sander! I had the same problem with some old kind of finish on my wood floors that turned liquid when I tried to sand it and then congealed on the sandpaper. It was just as bad with a drum sander as with an orbital one, except the drum belts cost $15 to $20 each--ruined in 15 seconds!!!
I had great success with a hand steamer used for cleaning around the house---steam the glue and quickly scrape it off. When the glue is mostly gone, then you can go to the sanding step.

Carla in Sac