1: Take out the ply.
2: Make sure the van floor pan is CLEAN
3: Scrub with abrasive paper to kill the gloss, wash and dry.
4: Mask the sides to allow painting up to about 4-6" - don't forget the doors.
5: Apply 2 coats with a special roller or stipple brush, don't leave any places where water can get contact with the steel. Allow the first coat to dry, preferably for 24 hrs before applying the second. Use a good respirator type mask or keep all the doors wide open and take frequent breaks in the fresh air.
Someone else may be able to advise how much you need, haven't done one for a few years so can't remember.
Someone else may tell you to leave the ply in place and paint over - fine until water finds its way underneath, then your unprotected steelwork will rot like pears on the orchard floor.