if it's the original 'Hammerite' i.e. pre 1985, then i think you have to bite the bullet and use hammerite thinners/cleaner. once AkzoNobel took them over, the formula was changed (health & safety ... fumes etc) and post 85 hammerite will clean off ok with a proprietary brush cleaner (least, that's what i always use.) i'm not that impressed with hammerite these days, and have found several makes of bare metal paint which seem to work better.