If I were you I'd treat any surface rust to a careful at first wire brush and angle grinder bashing - then some sort of anti-rust treatment and then POR15 it. For a top coat KBS coatings have a satin chassis top coat that can be sprayed or brushed on. I've used this on the sills on my W123.

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!