I've seen red oxide on aluminum and wondered wth put it on there.
Zinc chromate is the aircraft builder's and owner's best bet for
aluminum, magnesium, or chromoly.

If I were welding up a fuselage of 4130, I'd powdercoat it. The old
ones I see which are well-preserved are painted with zinc chromate and
then injected with boiled linseed oil.

I agree with this writer:

http://www.paintcenter.org/rj/feb05d.cfm