Surfacer, is a build up primer. The idea is to spray multiple layers of surfacer, then block sand it off. This helps to "straighten" the body for a smooth looking paint job. Sealer is a must also, it eliminates bleed through, and any differences between the primer , and finish coat.It also water proofs the primer. Always buy your entire paint system from the same company, either PPG, or Dupont, etc..