I have OSB subfloor that is not flat (eg in one corner of a room the subfloor drops 1/4 inch over 2 feet). I want to install vinyl plank flooring but I need to make the subfloor flat first. I will probably install a 1/4 inch sureply underlayment first. Even to install the underlayment, my subfloor needs to be flat. How would you do that? Sanding down the high spots? (this would weaken the subfloor though). Using a leveler? (most leveling compounds should not be used on OSB, as it is not a good substrate)