I think the "inside the tin" part is key.

I've carried a fire steel on my keychain for over a year and it looks like new (minus the ordinary scratches that come from normal use). No nail polish, no shrink tube, nothing. Just put it on my keychain and forgot about it. I use it, oh, a couple of times a month... either to start a BBQ grill or just play with. My pockets stay dry, and my dresser is dry (where the keychain spends 10 hours a day).

If you haven't already, you might try putting a silica gel pack or two in there with it. See if that makes a difference.