Paint peels because the surface gets damaged, and the undercoats / base
metal oxidizes. And so the peeling gets worse and the rust becomes more
pronounced.

The ONLY solution is to rub the affected region back to unoxidized paint and
if necessary, to bare metal. Apply an etch primer to the base metal, normal
primer to the rest, and apply 2 - 3 coats of colour to seal the surface.
Finally, heat it for 30 minutes or so with a heat lamp to convert the paint
molecules to hard long chains from the soft short chains.

There's no short cut. Just applying colour paint over an oxidized surface is
a waste of time and money.
Stewart Dibbs

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Damaged and repainted before you got it? Does that door match the rest
of the car?

'Curly'

That's a cheapo aftermarket paint job. The door was damaged in some way at
some time. The only cure is a repaint.