If you want to protect the bare metal the most effective treatment prior to painting is zinc phosphate tricky to apply.
spray on cover, with damp cloth for an hour or so rinse bare metal with distilled or deionized water first if possible. or slightly acidic water. zinc phosphate forms crystals sensitive to as change in Ph on the surface of substrate causes crystals to form. keep it damp so crystals can form grey to black in colour. Most body shops use wash primer rather than treatment.
spray on cover, with damp cloth for an hour or so rinse bare metal with distilled or deionized water first if possible. or slightly acidic water. zinc phosphate forms crystals sensitive to as change in Ph on the surface of substrate causes crystals to form. keep it damp so crystals can form grey to black in colour. Most body shops use wash primer rather than treatment.