I clean with soft scrub with bleach and then coat the deck and non-skid with Woody Wax. The softscrubb with bleach will kill the mildew and whiten the deck. It's easy, you clean, wet the deck and use a dedicated brush to brush on small amounts of Woody Wax over the entire area. You then rise and dry off with a towel buffing lightly until dry. It is not slick, but really beads water. Lasts two to three months and then you recoat. It will bead water when effective. I then spray Totally Awesome from a garden sprayer cut 10 to 1 and melt bird and spider droppings and rinse with a hose. The key to a mildew free boat deck is to keep it waxed and keep the pores full. Pressure washing is probably making your gelcoat more porous and causing you extra work.