Always, always, always
When applying any type of finish, READ the instructions on the tin! And follow them, they're there for a very good reason: to give you the best end result.
Didn't mean to shout, but lately on various DIY-forums we encounter the same 'problem' being posted: after sanding and applying two coats of HardWaxOil the floor looks patchy. When we then ask how soon after the first layer the second layer has been applied, most times the answer is: after 4 - 5 days.
Never, ever leave it that late between applying the two coats of HardWaxOil; all brands (be it Osmo, Blanchon or other) specifically write on their instructions: apply the second coat within 36 - 48 hours. Otherwise, the grain-hairs of the wood 'stick-up' and the floor needs a light sanding first.
Read the instructions, no matter what type of finish you plan to apply, and when still in doubt: call for advice - every proper supplier has either a company sticker with contact details on the tin, or the manufacturers help-line is printed on it.
Last sanding round
It is highly recommended to sand an unfinished floor after it has been installed. Not only to create a smooth surface when the boards feel rough, but also to remove greasy fingerprints and excess - spilled - glue. If you do not remove these they will definitely effect the end result of the finish.
If there are many height differences start with grit 80, if the floor feels smooth use 120.
For oil finishes: maximum grit to use in last sanding round = 120
For varnish/lacquer finishes: maximum grit to use in last sanding round = 150
For the best end result you'll have to use (hire) a professional belt-sander (and edge-sander). Like the description says a belt-sander has a continuous (or endless) belt, or rather continuous (or endless) sanding paper - called Belt cloth. These belts are very easy to 'wrap' around the drum of the sander (1 minute tops), but most importantly will give the smoothest results on your floor.
Most DIY hire centres can only supply you with the (much lighter) drum-sander. Drum sanders have sheets of sanding paper, that has to be wrapped around the drum and fixed firmly in place with a metal bar. This metal bar, when not attached correctly will create shatter marks all over your floor. The problem is that those shatter marks will mostly only show up AFTER you applied the finish product (varnish or HardWaxOil).
Trying to remove these marks with a rotary sander will create another problem: circular marks in your floor.
Restoring original floorboards or parquet floors
More and more original wooden floors are (re) discovered underneath carpets. Restoring these usually also means sanding off the old finish layer.
Pleas check our wood guide on repairing/restoring your original floor for more tips and advice.