Regarding your poly primer question... For nearly all paints, you either need to apply them during their "open time/recoat window", when they are still not completely cured and can accept a wet coat of another paint and make a chemical bond. Or you need to wait for the complete cure (which is often very fast) and then sand (to create a rough tooth, for adhesion) and then spray. The open times are all over the board but often vary from just a couple hours to maybe 12 or 24 hours. Epoxy primer is the exception (in my experience). I have used SPI expoxy primer which has an open recoat window of about 5 days. I also use Kirker Enduro Prime which has an open time of at least a few days. The downside to these epoxy primers is that if you spray and need to sand, you need to wait at least a couple days and maybe up to 5 days because otherwise they will load the paper and make a big mess.
Poly primer has a lot of talc, like bondo. It will absorb moisture and is best dry sanded. Or wet sanded with a day or two of dry time. Urethane primer is a two part urethane product and can be wet sanded, briefly dried, and recoated. It does not absorb water and will seal a surface so you need not worry about rust. Many primers don't like outdoor exposure to UV light and will break down in UV. Epoxy typically isn't UV resistant and should not be exposed to the sun for long (weeks) periods of time. A couple days will just help it cure. Rattle can primers are all pretty porous and will absorb water and rust through.
Have fun painting. Epoxy primer can also be brushed on. That works great if you just want to seal your bare metal and come back to the repair later. Some people swear that epoxy primer is needed to seal the bare metal before filler is applied. There are diverse opinions about that - I put my filler on bare metal, per the directions on the can. Epoxy primer has a long pot life (in mild temps, you can mix and shoot it for a couple days). Other primers are more like 30 minutes (poly) to 1 hour (urethane).
Check our Southern Polyurethanes for Epoxy Primer. Barry is owner. He answers his phone all day and is a huge tech help. I used his clear on my car and thought it was a great product for a home paint job. Also very economical when compared to PPG and other big names. Kirker Enduro Prime is also good epoxy primer and you can buy that on line. It is very economical. Rumors have it that Eastwood's epoxy is actually Kirker, repackaged. I really like SPI's urethane primer. Put a couple coats of that on a nearly finished panel. Wait an hour. Apply guide coat, and sand to 400. It is great stuff.
Best of luck. That is the fun part!
Pat