Before I laid the new floor, I spent some time cleaning up the frame underneath and treating the areas of surface rust I found there.
I also had a mate of mine come out with huis welder and weld in some extra steel beams into the existing frame, mostly across the aisle and the large spans on the sides. This gave me a much better framework to support the smaller sheets of plywood I would be laying down.
I measured and cut all the sheets of plywood and sealed all the cut edges with a waterproofing bitumen paint.
I also took the time to create a detailed map of the underfloor frame while it was fully exposed, because I knew it would be a tremendous help later on when it was time to screw down gthe new floor.