Hello,
I am a newbie. I am working on birch floors (confirmed with flooring specialist), in my upper two bedrooms, my goal was to achieve an Expresso color or close to it.
PROBLEM:
Left the dealer with my stain in an enclosed box got it home, put it to the side, completed my sanding starting with a 36grit to 120 grit, vac & tack clothed 3x's, used wood conditoned, applied the wrong color of stain inside the closet (test area), 5 minutes goes by (using a stop watch) I wipe off the access stain, wow this is really dark, the dealer gave me Ebony. I later returned it for Jacobean and purchased a lambs wools applicator at the sametime. I always like to ask questions, I guess i was tired and left the store, with a few free products.

1st coat of stain: I wiped off in 5 minutes, let dry. It looked like a bloched leopard.
2nd coat of stain: Iwiped off in 15 mintues, let dry. Not only is it a bloched version of the first application, it's darker, and it now has little fibrious hairs in various areas of the floor from the lambs wool applicator (came no instructions ) these products were purchased (from a very well known business in Ottawa that caters to contractors for years and years ) in a freeser bag with a label on the front of it lambs Wool applicator, no instructions. I always ask questions, at this point I was exhausted and forgot to ask do i prewash or rinse before using?? ..anyway..
3rd coat of stain. I got down and tried to remove as much of the little fibres as possible, applied the stain and let it dry, it went darker with alot of brown coming through, i kinda new that would happen.
4th coat if stain, looks great!!! Darker than what i expected , but, boy what a lot of prep work.

Next Step: Water Base vs Oil. I decided to go with water base urathen for it's drying time, it's doesn 't yellow, and I can be in the house. My next step is to apply my first coat of urathen, and if I understand correct, I would need to sand lightly using 220 grit, then apply my final coat . My question, once the final coat of urathen has dried do I need to put a sealer on top of that as well??

if you stained the floor 4 times I would be scared that the finish is going to peel like a banana. Stain bonds to the wood, it wil not bond to itself that well at all.

Kevin made a good point.You have saturated the grain with 4 coats of stain. You had better give it quite a few days to dry before attempting to put any kind of finish on it.

IF you make it to your final coat without peeling and delamination issues, , you simply let your last coat dry and you are done.Nothing goes on top of it ,especially not sealer.