Magnesium alloy wheels develop what's called white rust, and this generally looks like little worms have gotten under the paint. This oxidation can work its way into the metal and cause cracks.

Currently the best coating for them is dichromate, however it's toxic and is being phased out. Aircraft parts are still coated this way, and sometimes you can track down aero engineers who will do it for you.

Someone??? said that the magnesium content on the magteks is not very high, which probably makes sense given the un substantial weight differences, so this level of preparation may not be required.

I have an old set of Campags that the paint has been well maintained on, and no signs of worms, so as long as you keep the surface prep maintained you should be OK with two pac.