I've always been in favor of a Metallic form, but mine is more complicated. I was thinking an active power -- your 'steel skin' would have it's own Armor variable. Example:
I spawn with the Steel Skin active power. I have 0 Armor, 100 health. I pick up 25 armor, then I activate Steel Skin -- I become hyper dense, bullet proof, and my armor jumps to 500 (or some other number, most likely lower, for balance reasons). If someone decides to pound on me with rockets or some other weapon I'm vulnerable to, my armor is reduced (lets say they hit it down to 250). If I were to deactivate Steel Skin, my armor would go back to 25 (what it was before I activated it), I am no longer bullet proof or hyper dense, and I must wait X amount of seconds to reactivate it.
I can now gather armor for myself for when I do not have steel skin. If I activate steel skin again (putting my armor back at the previous number of 250, what I had when I deactivated it) and somebody reduces my armor to 0, my steel skin is demolished. I must wait 30 seconds to 1 minute until I can use it again, and I am now using my previous armor. Steel skin can be pierced by the Armor Piercing super power, thus giving the power a weakness.