Salt works fine as a masking medium for rusted areas, esp. at large scales like 1:35. Prepare the area with rust-colored paint, let it thoroughly dry. Then wet the area carefully with water (a drop of APC helps) and then shake some salt into it. Let thoroughly dry and then paint the model as usual - works best with a paintbrush, but this also works with a standard brush when you are careful. Once dry you rub off the salt, and the rust is revealed - plus some texture that looks very much like corroded steel plate.
I've tried this stunt in 1:72 (on both vehicles and aircraft, for a chipped paint look) but IMHO it the results were rather messy and did not look good, the effect is too coarse. Hairspray - though more difficult to apply - might yield better results at small scales. Personally, I have changed to lay down a metallic primer base, then apply the standard surface color, and finally thin the wet paint with an appropriate solvent and/or do wet-sanding to the area. The effect is similar, but I have better control over what's going on, also because I do not have/use an airbrush.
