I am refinishing a floor on a house I'm fixing up. This is my first time doing a floor and leaving it "natural" instead of using stain. Someone has tried to sand the floor before, and in one room there are horrible drum sander marks I was able to make look a bit better, but not great. The other room and hallway look better.
My issue is with sanding sealer. The polyurethane I bought says that if you are using it on natural white oak, you should use sanding sealer first. What a pita this product has been!
It's my first time using it, and I sort of assumed it would look like wood conditioner...Um it doesn't. The label didn't say anything about how to apply or it leaving marks so I went on my way and applied it without a ton of care to how it was going on. It left applicator marks on the floor, it looks uneven and I about hyperventilated when I saw how it dried. It really highlights the divot marks, and there were very clear start and stop lines from the applicator
I did my best to sand (by hand) to smooth out the dark start/stop lines and then did another coat to try to smooth it out.
One thing I noticed is how much this raised the grain (terrible!!) After I've done multiple passes with multiple Sanders, the baby-smooth floor now feels like it has a rough 5 o'clock shadow. I could cry!
Am I supposed to be using an electric sander for this? The directions on the can stated it just needed to be "roughed up" for adhesion purposes, but I can't imagine how this horribly rough floor will now be with polyurethane. I'd hate to have to sand that down and redo it.
I tried a little test area and it seems rough (almost looks like mini little dust or bubbles) in areas over the sand and seal. Someone told me that's normal with poly and it will be unnoticeable in another 2 coats. Is that accurate? Any tips/tricks/advice for the headache that is sanding sealer?

Some things that may help you. Are you applying the sealer in a draft free room, then opening it up for ventilation after it is tack free? This will help the sealer flow and even out the thickness of the finish.
What sandpaper did you finish the sanding with. Did you use a sanding screen under a buffer to cut any "fuzz" left from the sanding process. Any fibers that have not been cut right to the surface from a coarse or sharp sandpaper will absorb the sealer and as they dry will stand up. Sharp sandpaper will leave fuzz.
The grain raise as it is called will be able to be cut to the surface with fine sandpaper easier with a sealer compared to a finish. The sealer dries hard sooner and is made to powder up as it gets abraded smooth. You can use a random orbit sander to help cut the grain raise. We normally use a used 100 grit paper, but with a smoother surface sanding with a 150 grit paper will work. The sealer is made to soak into the surface as it is drying, so even if you think that you are sanding too deeply, there should be enough sealer left to be a good base for the finish coats, which are made to be more flexible and stretch with normal embossing that takes place over time and not chip from things dropping on it. A too thick finish is more susceptible to chipping, or fracturing when it is stressed

Thank you as always, Pete! I got the first coat of poly on and it doesn't look terrible, though I am unsatisfied with divot spots in some areas. See below.

https://ibb.co/KWzy9s1
https://ibb.co/bNvgVzq
https://ibb.co/MV1kWTn
https://ibb.co/CKhrYDd

A matte finish will help diminish the reflection so you will not notice the surface. It looks pretty good to me!