Hello everyone!
I repaired my car and replaced the hood and the fender. The hood came from a gray car which I sanded down and primed in gray. The fender, black, is sanded and not primed. My car is orient blue metallic. We were able to find a super close match with the color which I'm very happy with.
At this point I'm ready to test on the old panel before I touch the car.
Should I prime the entire fender too?
Should I just prime the edges adjacent to the hood?
Do I need to use some sort of sealer before I lay the base coat?

I'm afraid the primer is going to bleed through.
Any suggestions? Here's a few pics:

https://ibb.co/3SwsWkF

https://ibb.co/1Z54j13

https://ibb.co/tcHQ5Fm

Thank you.

Why would the primer bleed through?

Your new basecoat will adhere just fine to previously painted surface sanded with P800 wet.

What I would suggest is that you do a single closed coat of dark grey to black sealer or epoxy first. This will make your base coat colour consistent across the join between the guard and the bonnet.

Thanks. So in other words I can paint over the black fender once sanded with p800 grit and give a coat of dark gray primer sealer to the hood/bonnet

Yes, assuming the paint on the guard is in good condition. The dark sealer is just to help you get an even colour match at the join.