Painting wheel arch spats

Does anybody know the best way to paints the spats so the paint does not crack. I am spraying with basecoat clear coat PPG paint and almost at the point of painting these items. With the spats being made of flexi material i don't want to paint them only to find the paint cracks as soon as they get bumped neither do i want them to stay in black.
Any ideas? do you use a different hardener and primer?

Plastic bumper filler, paint and plastic (filler) primer. All available from Halfrauds.