Primer — sold in the model world as “surfacer” — is the undercoat that does two jobs at once: it grips bare plastic so paint can bond, and it fills micro-scratches while revealing any flaw you missed. Sprayed over sanded seams, a gray surfacer is the moment of truth: every remaining blemish shows.
Numbered surfacers (500, 1000, 1500) indicate coarseness — lower numbers fill more, higher numbers spray finer. Most kit work lives at 1000–1500.
See it in context in our paints & primers guides, or explore the tools hands-on in The Bench.