I use real 2-part epoxy (not the quick kind) and fiberglass matt on
things that are not structural or very visible. It can be thin or built up for strength.
Of course, any rust has to be removed and treated as standard prep.

Real epoxy does a great job of sealing and stopping any further rusting.
Practice helps - It can be messy if you do it wrong.
Uncured epoxy cleans up with alcohol.

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1985 300TD 1981 Scirocco 1.6D conv 1986 Golf 1.6D 2003 Golf TDI