For rust on chrome, I use "The Must for Rust" on small parts and just let them soak in it for a couple of hours. For larger things like fenders, I like to use naval jelly. (Both of these are basically phosphoric acid, wear gloves!) I put it on with a blue, no-scratch scotchbrite pad, let it set for 5-10 minutes coated in the naval jelly, and then scrub it all over again with the blue scotch-Britte pad. After either the soak or the scrub, I rinse the part with lacquer thinner, and then follow up with Mother's alloy polish. This method avoids adding any scratches to the chrome surface, and the Mother's gives some protection from recurring rust. I just did a fender this way that really amazed me when it was done, because it had quite a bit of rust. Its not perfect, but from 5ft away you couldn't tell it.
Matt