The side panels are already painted... remove all decals and residue... Clean them well with soap and hot water... then wax/grease remover...Then scuff them up, nook and cranny, with scotch brite (but dont "burn" through the paint" and then paint them with a paint with some flex additive in it or just use U-Tech paint that you can get from a body shop supply... With the black topcoat... I really don't see a need to prime them as long as you dont wear through the paint that is already on the the panels.
It is actually cheaper, faster and better to do the prep yourself and then take them to a local body shop for a "cash" deal. Professional products... professional experience/application.
Rattle can paint will not be very durable.
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