I think prepping it is the key to having the paint last. After removing the shield from the car, I used a product called Auto Prep made by Clean-Strip to remove any oils or road film. I then lightly sanded everything for a bonding surface for the paint to adhere to. I then cleaned it again with the Auto prep to remove any of the sanding residue. I then sprayed on thin coats of the Engine Enamel waiting 15 minutes between coats. I do not remember how many coats I applied, but the can calls for 2 thin coats followed by a medium coat, which sounds about what I did. I let it cure for a week before reinstalling it. I also while the bolts were out, sprayed them too to match the shield.
Hope that helps!