That's exactly what I've been doing and I'm quite happy with the finish so far as it is still a tool.I've wondered about those generic hardeners too. Usually I'm loathe to mix chemicals from different brands / paint systems because the paint manufacturers are always messing with the recipe, sometimes for environmental reasons & sometimes just because. But I've read the same thing specific to Tremclad. I painted my mill stand with Tremclad & used a disposable foam roller & foam brush for hard to reach areas. It is really easy to apply & leaves a consistent finish vs brushes.
If I had a machine I really cared about, I'd go Endura. Its meant to be sprayed, also high in isocyanate, but brutally tough stuff.
I'm honestly not a painter and I'm not expecting perfection with this. if I was I would probably have a professional do it.
Started on the work bench today.