First off, I'm going to assume all the parts are plastic.
Make sure that all the parts are CLEAN. Wash with a good dishsoap, even let them soak overnight. I like to use a good citrus cleaner in case someone has been Armorall happy in the past.
Once cleaned, you can either use a spray can primer that is meant for plastic parts, or a spray on adhesion promoter. I went with the adhesion promoter. Once sprayed on, it actually softens the plastic. When the paint of your choice is sprayed over top, it helps it cling.
Most auto body supply places should be able to help you out.
And I don't want this to sound uppity, but FOLLOW DIRECTIONS on the products that you use. Very important!
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