Stu,
What ever you do, DO NOT use Case/IH Irongard and Rustoleum rusty metal primer on the same item. They are not compatible. The Rustoleum auto primer (grey) will work fine, but not the rusty metal primer.
I tried it on some parts that were rusted and pitted. These parts were sandblasted and wiped down with a tack cloth. I also use rubber gloves in order to not get any oil from my skin on the metal. While the metal was good and clean, I applied the primer.
When I painted these parts, the paint started separating, creating a big mess.
I had to let them dry, sandblast them back down to raw metal, and start all over.
I use Irongard in the quart, and mix what I need. This paint is expensive, and I was not happy when I had to waste the paint because of the rusty metal primer.