For many years I used galvafroid as a cold galvanized undercoat, never had a problem with it, though they didn't do it in spray cans way back then:embarrassed:.
More recently I hand painted it onto the metal gate at work, still looks tidy and rust free three years later without any top coat.
http://www.dstgroup.co.uk/index/products/view_products/139/
Regards rog8811