The Colourbond paint will be turps-based enamel, while the PUP beneath it will be Xylene/Toluene based 2 pack. Many solvents or cleaners will work better on removing the enamel than the 2 pack (ie the Graf remover or Acetone), but if the rattlecan paint is really that badly bonded, why not just try a high-pressure water washer? (A real HP washer, not a Soopacheap $99 mud-rinser)
There are other threads on prepping for painting over PUP, but they essentially boil down to ensuring there is no dust to breathe, so wet sanding/wet cloths to wipe down and disposing of cloths while still damp into plastic bags. Personal Protective Equipment is a must, gloves, covered skin, mask.
