Ive not seen Hammerite here but that doesn't mean you cant get good metal paint..
I use epoxy red primer and epoxy gloss or enamel as they call it here..Cant remember brand name but I can check if you like.
To test it you can paint it on a plastic bag..Lets say three coats and let it cure... Scrumple up the bag tight and reopen... Paint does not flake or peel at all.
For rusty metal you can buy rust converter pretty cheaply...Apply 24 hours before primer..
Here I am talking about bloody paint again!!!! Hate the stuff!