Instead of magnesium, I usually use a little fatwood when the wood is really wet. Of course, preparation is the key. On our last camping trip, I just made a bigger tinder bundle using eucalyptus bark. I love that stuff. I didn't use any fatwood despite the wet conditions. Eucalyptus bark ,feather sticks and split wood was enough to get the fire going great pretty fast.

When I see eucalyptus trees, I note the spot and gather some bark before heading out. But I always carry some fatwood in my fire starting kit.