It seems that your door was only UPVC coated.
Take the advice of ianmunt for preparation of the door, prior to painting with a water based type paint. I would use any outdoor plastic emulsion type paint; far better than oil based paints which always end-up flaking.
In fact I have used indoor satin emulsion on a section of ranch fence and after many years it's still in good condition ( Perhaps paint manufacturers are 'having us on' when it comes to what paint is best for individual surfaces; be they indoor or outdoor.)
Best wishes ...Ron