Nothing is being protected. The zinc has a lower voltage potential than iron, thus more available electrons, so it gives up electrons. Electro-galvanic corrosion is all about which is the more noble metal, which has more electrons in a paring, magnesium is worse still. Whenever two metals are togeter it needs a dielectric coupling of some sorts, a rubber gasket etc., the Navy has extensive experience and technical papers on corrosion if the subject interests you further.

In cars, it is usually best to remove any rusted metal and start from there, it is extremely difficult to stop rust. Some treatments such as Naval Jelly will remove substantially all of the rust with an acid etching process, and I have had some luck with "rust converter" paint-on treatments if the rust is all exposed and cleaned first. Sandblasting and cutting is the normal treatment when restoring old automobiles and boats however, complete and permanent.

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Gone to the dark side

- Jeff