Try white vinegar and hydrogen peroxide 50/50 mix with a couple of drops of dish detergent. Works great on fabric and fiberglass.
This is actually a reasonably dangerous practice, generating peracetic acid. This is NOT just vinegar and H2O2, this is much more reactive. OSHA has not yet established a safe exposure limit, but sub-PPM is expected. Read up on proper dilutions before proceeding. Certainly not something I would slop liberally around the deck without doing the reading. Sure, some folks get away with it. Some folks mix ammonia and bleach. But please, don't believe me, Google "peracetic acid."
A safer approach, if you like peroxide, would be to just use Oxiclean. Less aggressive, but MUCH safer.
And it is a myth that vinegar will keep mildew away if subject to rinsing. The pH rises and it just becomes food, growing mildew faster. Mildew likes a weak acid environment.