Salt water works - mix up a mild salt mixture in a trigger bottle and go to work, let it dry in sunlight, then repeat. Afterwards you need to wash it down with clean water, and it might be worth wrapping any metal work that might be hit before you start.
Caveat, I have no idea whether this affects weatherproofing, but it worked beautifully to solve a minor mould problem on mine (12oz Aus canvas) and I've since had it out in a reasonable shower with no water ingress through the canvas.