I have refinished all of my hard wood floors in my house and can help you here...
I finished the dinning room (150sq ft) with a machine sander then 220 grit (palm sander), vaccum, vaccum, vaccum, then used a tack cloth to get what vaccums can't. Very , very important.
I used an el cheapo sponge brush and applied an even THIN coat. (using a tack cloth again on the area I'm about to coat) then move on to the next area or run and repeat process overlapping the light THIN coats.
next you must sand the light coat (scuff it up so the next coat can penetrate and stick) I used 220 grit and a palm sander rather fast (you just want to scuff even not take it off)
After you do this about 4 or 5 times you will have your basketball court :))
It is an awefull lot of work as I can testify, but this is how you get your glass ice look!.
Brushes and rollers are trapping too much air coupled with a thick coat, when spread the air pockets have no option but to flow from brush or roller into the finish. (stirs up the air as applied)
I hope this will help you!
To get the Trapped air pockets out of your finsh (you may have to sand it all off (only the finish not the stain)and start over... Could be alot of work..
This happened to me in the living area when I used a sponge to apply first 2 coats then a roller for a fat third coat (okay finish but air trapped in final coat... (what was I not thinking? oh yeah, speed as all the thin coats and tack cloth and sanding in between coats is very TIME CONSUMING)
So I had to take off the topcoat using 100 grit down to 220 grit with a palm sander (what I had) could've used a sheet sander goes way way faster cause it's alot larger.
THE TRICK IS LOTS AND LOTS OF THIN COATS WITH A LIGHT SAND IN BETWEEN COATS. IT WILL ABSORB AND BUILD UP. SEALING, PENETRATING, AND PROTECTING YOUR FLOOR FOR A LONG LONG TIME :)
600 Grit is for removing scraches. You do not need; However, if you do , A 600 grit sand after the 220 grit and also inbetween coats would be astonishing finish (my finish is all 220 and its smooth as glass with 4 to 5 light coats