Some primers can be used onto a variety of different base material and some are for very specific tasks.  They are really important so don't miss them out. Primers are the foundation of a sucessful paint system and are designed to bond strongly to the base substrate and the subsequent paint layers. They help to make make the overall finish thicker, tougher and more chip-proof. An important primer tip - stick wherever possible with the manufacturers' system to avoid any unexpected incompatibility between coats.