The following recipe was provided by Paul at Metalcrafters in Phoenix, AZ. Warning: Do not attempt this without proper ventilation and protective clothing.
We did a large project for a restaurant last year. Granted it was out of mild steel, but the prints called for everything to be "rusted and sealed".
We accomplished this by spraying (from a spray bottle) muriatic acid, full strength, onto the steel.
Wait a while.
Then we sprayed chlorinating liquid (pool supplies) on. Wait a while.
Hose it all off, and TA-DA! Instant rust.
People that saw it couldn't tell it from rust that was ten years old. The whole process takes about four hours.
CAUTION: Acid can really tear you up, so watch it!!!!!!!!!!!