With rust it is best to cut it out and weld it up, then grind the weld down, fill with putty,sand, prime hen paint.
Your spoiler without actually seeing it, I would say that the clear is comoing off if it is metallic. I would sand it back and re paint with acylic then clear it a month later, (it is best to let acrylic cure). acylic is easy to use and it you screw it up you can rub it out and start again, also acylic is almost dry as soon as it comes out of the gun. This means don't worry too much about overspray as you can just sweep it up.