Welcome to the site Liam!
What color is the paint in trouble? If this is picking on ONE fender or area it might be a lousy repaint or a non original replacment panel and you might never win the battle but can fight as best you can.
I've had outstanding luck here in "RustoChusetts" on steel wheels using Rustoleum's "Hammered" finish paints either as the finish or the primer. It sticks and is self priming!
It's sold in both sprays and liquid by the qt anyway - don't know if smaller container is available. I'd use this first and top coat with color needed or if lucks to find this in a color that works for inner lip of a fender well this would look fine by itself.
It's a battle as said and this might buy the most time between redoing it again,
T
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Tom Greenleaf - MetroWest, Boston