Some food for thought. I'm going to launch a few tests with Plasti-Dip over magnesium bars, see how that works.

I was recently on a SAR call (actually recovery, not rescue) and found myself with my oversized fanny pack full of emergency & survival supplies IN the water for about 3 hours. When I got home, it was a bit of a shock to see how much stuff I was "sure" was in a waterproof over-wrap got wet. My first aid gear bore the brunt of it, and has been relocated to a Small (but heavy!) Pelican case.
I've found that good old WD-40 just plain works to keep the wet off the metal.

Fungus on the Magnesium (and everything else) is solved with a solution of OxyClean. This stuff is amazing, it's the WD-40 of cleaners, and is fairly benign to most fabrics.