I used vinegar with good results.

schreef in bericht
oups.com...
I have a foldable kayak with a interior wood frame. Having left water
in the boat for too long, a number of pieces have what looks like
mildew stain and some minor rotting. The integrity of the pieces are
fine and I am not concerned with the aesthetics. I would like to
refinish the pieces with spar varnish but I am wondering if I need to
get the mildew stains out and how to do that. A light sanding smooths
out the pot marks but doesn't get the stains out. Any suggestions??