I replaced the water pump on my wifes 300D a few months ago. Its now covered in rust. I am about to replace my water pump on my 300CD. Should I paint it first? If so what paint would you recommend?

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'84 300CD Turbo 132k (Anthracite Grey) - WVO - My daily driver - Recently named coo-coo-coupe by my daughter.
'84 300D Turbo 240k (Anthracite Grey) - Garage Queen
'83 300D Turbo 220k (Orient Red) - WVO - Wifes daily driver

I'm not a certified mechanic, but I did stay at a HolidayInn Express last night.