Design Nomenclature / Physical Model Making
Primer
Also called surfacer, high-build primer, filler primer, undercoat
A coating formulated to stick to the substrate, be stuck to, and sand. It is neither paint nor filler: high-build primer fills tool marks a few thousandths deep and no more. Its real job is to make one optical surface out of five different materials, so the colour coat reads as a single object.
In practice
"Primer will hide that" is the most expensive sentence in a model shop. Primer reveals; it is how you find out what the surface actually is.