If you have ever sanded a floor and proceeded to apply waterbourne Lacquer(Varnish), you most certainly would have noticed how grainy the floor surface becomes, Often described as a more natural finish, truth is, that grainy rigid look is actually grain raise, which when your trying to get a smooth long lasting finish is not good.

Variety's of varnishes and types of wood certainly come into the equation, some waterbourne polyurethanes definitely cause more grain raise than others, i suppose it depends on the solvent content, but with low VOC content and environmentally friendliness being foremost, it is certainly not going to get any easier to eliminate completely.

I've found Oak to be the most problematic to work with, Although a hard wood it does not react well to being soaked in water, Some types of pine are also pretty prone to repeat grain raise too.

The main part of the problem is with the machinery used to sand the floor in the first place, belt sanders tend to leave the grain open, therefore letting the water (from the varnish) penetrate deep into the grain causing the grain to expand and rise above the coat of varnish that has just been applied, which makes it near impossible to get a nice smooth finish on the second and then third coat, unless of course you keep sanding right back between the coats, thus leaving less varnish on the floor.

You could sand the first coat to a really high grit but really wood should only be sanded to a 120 grit otherwise you can get adhesion problems which could cause peeling of the varnish.

Best way i have found is to rectify this before applying the first coat, Simply sand your floor to a 120 grit with the belt sanding machine (hummell etc) then either Screen with a 120 carbon screen on the buffer or use a 120 grit on a palm sander or if your lazy like yours truly and have all the kit, Use a lagla trio or any rotating sander, the circular motion of the forementioned will close the grain down and stop the water soaking down so deep into the grain, Apply the first coat ideally with a brush or paint pad trying not to leave too much residue, this will make the varnish dry much faster and not give as much time for the varnish to penetrate the wood.
You will need to lightly de-nib the first coat, vacuum again and apply the second coat with a mop or roller, this time be extra liberal with the varnish (put on nice and thick)
Let the second coat dry, normally a good hour (depending on humidity etc) you will then need to quickly de nib once more but just lightly (by hand is best) with a 120 grit, You will notice that there's hardly any roughness on the second coat, but its best to give a quick de-nib, just to make it extra smooth. Finally vacuum again and apply your 3rd coat with a roller or mop and leave to dry.
You should now have a floor which is nice and smooth to the touch.