I think he means in comparison to butted mail. If the mail isn't riveted, you need a lot more steel to make the rings strong, so a lot of time it'll be made with 14 gauge wire. Boy does that get heavy. A hauberk can be over 40 pounds.
But yeah, the mail will of course be susceptible to rust after taking the grease off, but you can use something clear in color. Spray-on WD40 works, but it evaporates relatively quickly, so it's only good for a few weeks maybe. I've never re-sealed my voiders or fauld, and the only rust I have so far is on the ends of the voiders where it comes down near my wrists (and receives sweat more directly). The nice thing about mail is that when it moves, it scrapes some of the rust off on its own. When it accumulates a lot of it, you can throw it in a sack with sand, clean kitty-litter, or wood-chips, and slosh it around to scrape most of it off.
