Get it as smooth as you can with the sanding. I don't think you can use a pad under 3/4" flooring. Your best bet is get it as flat as you can and put a plywood rated for underlayment on top of that. Underlayment plywood has no voids to give or flex like Luan or reguar 1/4" plywood will. You can put a paper or even roofing felt under it. I think the proper installation of that wood will be either nail or glue. If you glue, you shouldn't use anything but the underlayment under the wood. Glue wouldn't stick. Your best bet would be go to the manufacturer's website and look for installation instructions or help. If you can't find that, try to get a number. Flooring manufacturers look for any reason to dishonor a claim for adjustment. One thing done wrong and even if it has nothing to do with the defect, and they will usually disallow the claim. I laid floors for @ 15 years, but have been retired for 5 and I have never bought a floor because I made sure to do everything the way they said. When I didn't agree with them, I got pictures and kept them. You can't go wrong doing what they say...just keep records that you did it their way.

We're at the framing stage now. At demo, the guys pulled up 2 layers of linoleum in our 1930 Spanish to reveal tongue-and-groove wood floors. The GC says they are fir--not quite hardwood, but a perfectly serviceable softwood. IF the floor lady can sand through all of the adhesive, he thinks they will look beautiful, if a bit rustic. (There are some loose joints.) If we can't salvage the fir, then they'll have to lay plywood and put new wood on top of that. The floor will have to come up at least an inch unless we rip everything out and start over. Hope not. The rest of the house has red oak with dark walnut accents.

I live in New York City and this is a 120 year old brownstone and we left the original hardwood floors intact and just refinished them except for where we removed a wall between kitchen and dining room (and added three strips of wood where the wall had been - the island is installed in the middle of where the wall used to be - and we also installed about 10 partial strips of wood where we removed a dumbwaiter and some ducting to make the kitchen about 25" deeper which is where we placed the fridge. here is a pic for context - there has definitely not been temperature control since installation - being an old house, it can get pretty cold in the winter and pretty hot in the warmer weather - we don't have central air, but do have centralized heating via boiler ( and thermostat placed on the parlor floor where the new kitchen is) As for whether these are carpenter ants, I googled big black ants and found pics of carpenter ants and that is what we have. Whether they are doing anything problematic, I don't know. Another forum poster said to check where the steel deck is anchored at the bric of the house because if water is getting in, that is a perfect place for a carpenter ant infestation. So I'm going to investigate that as a priority although I don't know who might be an expert on that It might be worth mentioning that the door to the deck used to be a window so we had a window converted to a door in addition to adding the deck... So a number of places where perhaps the changes could end up with structural issues and/or risk of new places of entry from the outside, for ants

I might have set the face nails deeper and used filler in the nail holes and staple holes but definitely not between the boards. It is just going to pop out and floors look awful when the filler starts falling out. I also might have tried to bleach some of the stains to see if they would become less noticeable. Unless the filler is well matched and applied sparingly, the filler may just highlight the flaws. However, those are issues that a knowledgeable DIY or flooring person can address when they are working on their own house. As a remodeling contractor or a floor refinisher, those tasks require a fair amount of time and effort for unknown results that likely don't justify the expenditure. When working in an old house, the goal should be an appropriate appearance, not new. Old houses dont look right filled with plastic finishes and vinyl windows and acrylic tub surrounds.