I used a material in my work years ago that we called "Buffalo Sxxt" that was designed for hand coating of underground piping.
Called Wax Tape, made by a company called Trenton.
Later, when I wanted to bury "something" I called their local rep in Houston, and was able to buy a small case of 6-6" rolls for cash.
This stuff is amazing, it hand forms into a waterproof shell that reminds me of cosmoline.
After a couple of years, it comes right off and cleans up with either turps or light oil, and in a pinch GOJO hand cleaner works also. No corrosion at all.
Since the wax seeps into nooks and cranies (and helps protect that way), I would pack ammo in a separate waterproof box first.
http://www.trentoncorp.com/wax1.htm
Old link was no good. Here’s a new one that’s just fine:
http://trentoncorp.com/products/wax-tape-brand-anticorrosion-wraps/