I have a mild steel flat ring riveted shirt and aventail and use this on it:
http://www.rustoleum.com/CBGProduct.asp?pid=40
You can get it at Lowes or Home Depot. Note this is the rust converter, not the normal rustoleum.
Think lots of light coats. I lay it out on cardboard. Spray. Turn it over. Spray. Shake it. Spray. Turn it over. Spray. Repeat. Repeat, until you cover up all the metal. I use about a can and a half on an aventail. 2 1/2 cans on my small mail shirt.
I think initially, it looks too black. But like Sean says, the links tend to rub the finish off at the edges with wear, but not on the majority of the flats. I think that makes it look better. I fight in the aventail about twice a week and at GW and Pennsic. I don't worry about spilling drinking water on it.
IT IS ALMOST NO MAINTENANCE. I maybe touch the aventail up once a year by respraying. The mail shirt gets used less frequently. I have never touched it up in three years.
I have sprayed it directly on rusty mail and on unrusted mail. Works fine either way. If it is really oily now, like mine was initally, put it in the kitchen sink, squirt some palmolive dish liquid on it, and rub it around. Rinse it off in hot water and then let it dry before spraying. I'll try to post some pics so you can see what it looks like.