My white cedar deck is under some very large oak trees. Even though
I've scrubbed, bleached, washed and restained this deck, by this time
every year the surface is black.

This year I'm planning on belt sanding down the entire desk to get to
the wood and applying some type of protectant be it tinted or clear
sealer. I'd like to add a fungicide to the material to hopefully
reduce some of this black stuff longer into the season.

Does anyone have any recommendations for such an additive available at
home stores? All suggestions welcomed other than cutting down the
oaks.

Thanks