I've been painting all sorts of things just recently from new electroplated doors to rusty swivel housings.
I point I have noted with using hammerite is, it doesn't particularly cover other non hammerite finishes very well, unless you really "dawb" it on, then it still doesn't guarantee its going to stick for very long. The only finish it does cover is a very rough brushed rust kinda of finish - I guess thats the market its aimed at.
I have found a range of "tractor" paints that are directed at the agricultural vehicles, which are roller applied, but still give a relatively good finish, a lot more suited for your standard basecoat primers.