Coating exposed luan plywood with ??

The floor of my trailer consists of 2" of styrofoam compressed between a top layer and bottom layer of 3/16" luan plywood. The bottom layer of the floor was wrapped and glued with a tarp like material to protect it from the outside elements. Water got between the tarp material and the luan plywood and caused mold and some minor delamination of the plywood. I had to remove the tarp like material to get to the mold and glue the plywood. The water intrusion problem has been fixed, however, the bottom layer 3/16" of plywood is now exposed to the elements. My question is "What would be the best coating to waterproof the luan plywood to protect from any further damage? Any suggestions appreciated.