I'll ask again... What kind of prep did you give the parts you painted?
Just to clear things up... On PROPERLY prepared parts both Kyrlon Fusion and Rustoleum's spray for plastic will work very well with no other adhesion promoters or special treatment.
Here is what you need to do to have the paint stick AND look good.
1). Clean, clean, clean. Wash the parts in hot water and TSP. Use a coarse ScotchBrite pad to scrub the parts and hot water to rinse. keep the parts oil free after prepping, Scrub EVERY nook and crannie and get them clean.
2) Dry them well, I use a hair dryer if I amin a hurry. otherwise letting them air dry on towels will work. Hang them on wires for spraying.
3) Wipe them down with a good wax and grease remover just prior to shooting paint. Prep-Sol, Pre-Kleano, etc. keep your hands off once they are de-greased.
4) Heat the spray can in a tub or sink full of hot water. Helps the paint flow nicely after it's sprayed.
5) Hang the parts so no dust or schmutz can settle on them while drying.
The important thing to remeber is get the parts clean and keep them that way until you are spraying paint.
There is not a thing wrong with rattle can paint as long as the parts are prepped correctly. I sprayed some interior pieces on my show truck with Krylon Fusion right after it was introduced 13 years ago. There has been no issues with paint peeling, even on the rubber cup holder on top of the dash.
Mark