I have picked up some 3/8" B-C rated exterior plywood to add to my subfloor as a second layer before I lay down Ditra and tile. I noticed a number of 1.5 to 2 inch wide 1 ply deep knot holes over the surface of some of the pieces. Is this something I need to address before I started attaching it? Do the holes need to be filled? Should the side with the holes be laid face down or face up and have them filled with thinset and then the Ditra?