Thanks folks for all the useful info. Ive used smooth Hammerite on scraped down rusty scabs here and there, followed with a couple of coats of Dulux trade high gloss. that was 2 years ago and no rust or chips have reappeared, thats why I thought it might be ok for rusty scabs on the hull. I'm now glad I asked. I agree with you Julynian bitumen is a waste of time. The blacking I had done 2yrs ago lasted About 6 months, (if that) Ive recoated down to the waterline myself twice more since then. I thought I'd do it myself properly this time, when I get the boat out of the water, and will be looking up Steelbond, Steelguard, Rustoleum, Jotun and Bondsprimer.I just hope they are in my price range.
Cheers Paul