I was at a few paint seminars and the speakers all painted tractors. They all said to just use half the recommended hardener in your paint. Then use an epoxy sealer on the cast parts. This helps seal any pores in the castings. I have tried this and the paint has a brilliant shine still 11 years later. I still get comments on my paint job.
I put three coats of paint on my tractors and each coat I use a little more thinner. It leaves a better shine. You have to put hardener in each can of paint you spray also. I paint as early in the day as I can. Then I wet sand the sheet metal parts up to 500 grit wet/dry sand paper. Put a little dish water soap in with your sanding water. This will keep the particles from sticking to the sheet metal. You may have to wipe off with a wet rag sometimes. Then just rinse off with plain water. Each coat of paint after sanding the one before it will come out with more of a shine
The more people you talk to about paint the more confused you can become. As for the gray primer return it and use the white. Your paint won't stick to the gray as well as the white. In a few years it will peel off the gray. I've had quite a few body men tell me that and they were right. Be sure to wet sand your primer before putting paint on top of it.I usually put two coats of primer on my tractors. I've heard some guys say they use two different colored primers so when they sand it they know how far down they are sanding. I never tried it but it sounds logical.