A method I used on my Pulse Rifle:
1. Base coat
2. laquer/thinner based silver/metal spray paint (if it stinks a lot, then it's probably ok to use). Leave to cure THOROUGHLY.
3. Tamiya (or other) acrylic top coat.
Get yourself some REGULAR Ajax window cleaner (used as thinner when airbrushing some acrylics, like Tamiya's). Soak a cloth, sponge or piece of cotton in the window cleaner and rub it where you want the weathering. The Ajax will dissolve the acrylic top coat and reveal the silver underneath, which should resist the Ajax much longer (if it even affects it at all).
This method is also useful to achieve "stain" effects by adding a few drops of another color acrylic to the Ajax.
Hope that helps.