PrimerUpdated a month ago

Primer is designed to bond with unpainted surfaces like metal or plastic while color coats and tri coats are not.

Primer is needed when you are painting a surface that exposes bare substrate such as metal, plastic, carbon fiber, or polycarbonate. On these types of material, the presence of primer will make the paint hold or last better. Additionally, if your repair calls for a filler then you will need to apply primer before applying the paint. 

If you are unsure whether you need it, you are better off using it. Adding primer when it is not required will not negatively affect your finish.