Stretch,
Hammerite is not sold in our part of the world. But I often see it mentioned on UK sites. If you Google Hammerite vs POR15, it seems it is not an equivalent product. But really, I have no idea. Like many solvent based paints, use with care and make sure ventilation is good.

We can buy rust paints in box stores. For example, Tremclad or Rustoleum seem mainly aimed at kind of jobs you have used Hammerite for. Hard to know if Hammerite fits into that group, or is more like POR15. Hopefully someone else might know.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5