well, since you do want to paint this oxidized alum, I would do the following:
get some steel wool and scratch up the alum and clean it up.
get a good primer and prime the alum. two coats might be best. I like Zinzer BIN primer.
then apply your final coat, either latex or oil. the latex that ran on another home that you mentioned probably was due to improper drying time or humidity or rain after painting. This is very highly unusual for latex to do this. something went wrong here.
Use quality paint from a Paint Speciality Store, more expensive paint IS better than cheap paint.