Hi johnny are you sure the smooth Hammerite has silicon in it? I think I read on the net somewhere that the hammered version might ......but only to give the hammered effect but the basic paint is still solvent based, in New Zealand versions anyway.
I have had no problems painting over Hammerite smooth with other oil based paint and have used both the spray can and brush versions on two bikes now, with excellent results, but it really does run easily!