Cleaning the rear drain plenum on our 67 Newport Custom

What is the best way to clean up inside this drain box? Compressed air in one opening and a vacuum cleaner in the other?

Maybe even spraying in some anti-corrosion fluid that will cover the bottom half of it and keep it from rusting?

It's lived a dry life being a California car and now with Norwegian climate, rain and regular washing it will get wet and exposed to moisture. It's not destined to be a garage queen.

We are new to these cars and appreciate all help and advice you guys can...

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