Arwyn, I only ever used Hammerite once and the hammered finish does indeed have a variable thickness, and if not applied properly can lead to pin holing as you suggest. Whereas Smoothright does not pinhole although is manufactured by the same company and comes in similar looking tins, which may be where possible confusion arises.

I have both brush and spray painted Smoothrite without issue, though there may be many superior products out there, and in some cases perhaps only available to trade or in trade packs...or only in the UK..?