Maybe call the local paint shop where the primer came from, or where you plan on buying the enamel for the specs.
Well, since it will be Massey tractor paint, I will be selling the paint to myself! Sadly though, I do not, or had not, had to deal with this when I use it to repaint a tractor. I just scuff the surface of the tractor with 80, then 150 grit, and shoot the enamel straight onto the old paint. Any bare metal spots was coverd with NAPA's grey sandable primer before the paint went on. Never gave me a moments problem before. But like I said, this is on a truck and is a whole different ballgame. I WILL take your advice and remove all the other primers though!
Ultimately, I think I will take the 2 old doors I am replacing, and use them as a test bed before shooting the truck with anything!!