Consider taking the new panel to an auto body shop and have them strip, prime, and paint it with quality automotive products before installing.
If you need to make additional mounting holes, do so before stripping/painting.
Use stainless steel screws (as noted above). You might also be able to find plastic (Nylon) screws.
And put Nylon or rubber washers on each screw before mounting. If you can't find anything else, get small flat faucet washers at Lowe's or Home Depot.
Sticks and Bricks: Raleigh, NC
2008 Cherokee 38P: at Ivor, VA permanently