The subfloor is 3/4″ T&G plywood. An existing 1 1/2″ strip oak floor was removed, and 2 1/4″ grade 1 maple will replace it. The first 150 sq. ft of oak was pulled up with a crowbar. The previous owner had used construction adhesive and flooring nails to install it. The subfloor was badly damaged, so I had to cutout and replaced it. An expensive and time-consuming task.
With this experience, I decided the last 100 sq. ft of oak would be removed a different way. I used a power saw with a carbide-tooth blade and made cross-cuts 1 1/2″ apart. The old flooring was chiseled out piece-by-piece. The subfloor faired much better. There are some narrow sections where the top layer of ply was removed, and some places where there is still glue sticking to the floor. I will plane-off the glue, but would like some type of synthetic material that I can spread over the top layer voids. Something like car body putty, that will spread, adhere, and cure to a texture that can be sanded smooth comes to mind. Would there be something made just for this task?
I would hesitate to add anything to a nailing surface other than wood. You would best advised to add 1/2″ plywood, or if that is too thick try just 1/4″ underlay. If the 3/4″ plywood is sound and sturdy, all you really need is the thin plywood to smooth out the hollows. But if there are large hollows and the hardwood has to span a gap of more than 4-6″ you might consider installing the thicker 1/2″ plywood.
I’m sorry but there are no quick fix compounds that will hold a flooring nail. Any epoxy leveling compound spread over the whole floor will be more expensive than the plywood. And these can take quite a while to cure, and cause quite a toxic stink in the house. These not used in the professional wood floor industry (except for concrete), only by uninformed amateurs. And just look at the amateur mess you had to clean up.
But you know, as long as the gaps are not too big you can, in most cases lay the hardwood over this existing subfloor just the way it is. Just make sure that you don’t put a hardwood floor joint on these gaps in the subfloor, and be sure and nail the hardwood floor every 6-8″, crossing (at right angles) to the joists.