I've used Hammerite smooth on 2 bikes now, both in spray cans and brushed on, it's an art getting a decent finish with both, but possible. It also works well over primer like red oxide and petrol does not affect it.
On the bikes I dealt with rust before applying it, it's ability to use over rust and to prevent further rust I have found "inconsistent"
I've heard there are some less good versions in some countries due to regs on solvents etc I'm not sure if that's urban myth
but in NZ we get the one with the original solvents