Best shed stain for pressure treated pine plywood siding....

Hi there,

I live in Pacific Northwest. Had a shed built last November 2018 with 5/8" pressure treated pine plywood siding.  Unable to paint or stain due to wet climate so now has weathered and ready to finish.  My primary concern is mold/mildew prevention and best product to prevent water penetration of the siding.  No eaves on the shed so the winter rain here saturated the siding on the bottom and actually penetrated moisture inside to the sill plate and flooring.  Located in deep shade, under pine tree and behind cedars.  Only gets sun during late spring into fall on the west side.  Cedar trim, metal roof.  Wondering also if needs prepping before finishing.  Thanks so much for your excellent forums!