what he said, sounds like a combination of oxidation, crud and old juice. the coil would rust before the mesh, unless its super over oxidized and way over burned. even then it would take a lot of sitting in water.
most of the times the vapor will deposit condensation from the wick which is brownish and cruddy anyways. i get a dark fluid in my bolt heads after a few days and now that i think about it it could be mistaken for rusty fluid. but most times any brownish color is from the crud building up in the wick.
if its truly rusting it might mean there is some distilled water in the juice to keep it thinned.
