Why primer exists
Primer is for adhesion, not colour. The odd part is that the general literature is lukewarm about priming plastic, while every hobby source treats it as compulsory.
I primed my first model grey and thought the point was being able to see the detail. That is a side effect. The job primer is actually doing is holding your paint on.
What a primer is for
A primer adheres to the surface and forms “a binding layer that is better prepared to receive the paint”, which improves both adhesion and durability.*
Two words in that sentence carry it. Binding: the primer’s job is to be gripped by one thing and to grip another. Durability: this is about the paint still being there in a year, not about how it looks tonight.
The clearest illustration is metal, where paint applied without primer is prone to peeling.* The second reason is porosity: materials such as wood and concrete absorb the solvent out of the paint, which changes how it dries.*
The part that surprised me
Neither of those reasons is obviously about a plastic Space Marine.
The general literature is noticeably cooler on plastics than the hobby is. Most household plastics are not very porous, and priming them gets framed around wanting a drastic colour change or long-lasting protection.*
Meanwhile every hobby source I have ever read treats priming a miniature as non-negotiable. So there is a real gap between the two, and I want to be straight that I cannot close it from sources. The plausible bridge is that thin acrylic layers on a smooth, mould-released surface behave differently from household paint on a plastic panel. Plausible is not sourced, so it stays a guess and is labelled one.
What Games Workshop’s own practice tells you
The most useful clue is not about primer at all. GW says resin miniatures need washing and gentle scrubbing in warm soapy water before assembly and painting.†
Nobody washes a model for fun. That instruction only makes sense if something on the surface as it arrives would interfere with what goes on next, which is the same problem primer addresses from the other side: get the surface into a state where the next layer can grip it.
The other clue is that in Games Workshop’s own system, the undercoat is part of the paint system rather than a preparation step. Their Spray range basecoats in a single application,‡ and White Scar spray is described as “designed specifically for use with Contrast paints”.§
So “which primer” is not only an adhesion question. With Contrast it is a colour decision that changes the result, which is why people who undercoat black and then complain that Contrast looks muddy are describing a system used outside its stated conditions.
The short version
Primer is a binding layer, and its purpose is that the paint is still attached later. Grey is not the point.
The honest complication is that the general case for priming smooth plastic is weaker than the hobby’s certainty about it, and I could not find the source that resolves that. If you know of one, that is the sort of thing I would genuinely like to be sent.
- *Primer (paint)
Wikipedia
Backs What a primer is for, in general terms rather than hobby ones: it adheres to a surface and forms a binding layer better prepared to receive paint, improving adhesion and durability; paint applied to metal without primer is prone to peeling; and porous materials such as wood and concrete absorb the solvent out of paint, which is the other reason primer exists.
- †Old World Almanack – How To Get the Best out of Your Resin and Metal Models
Warhammer Community · 19 January 2024
Backs Games Workshop's own account of how resin and metal differ from plastic at the desk: both arrive with little spurs called flash left from casting, metal carries mould lines that need shaving, resin carries little shims in the gaps between details, resin miniatures need washing and gentle scrubbing in warm soapy water before assembly and painting, a mask is recommended when cutting or filing either, and both need superglue rather than plastic cement.
- ‡The Paint Range
Citadel Colour (Games Workshop)
Backs Games Workshop's own one-line statement of what each Citadel range is for: Base gives strong solid coats as a foundation, Layer goes on top to brighten or highlight, Shade flows into recesses to make shadows, Dry is for drybrushing, Contrast goes over a white basecoat to base, shade and layer in one go or to glaze and tint, Technical adds effects, and Spray basecoats in one application.
- §A New Era of Paints – New Contrast Colours, Reformulated Shades, and our Best White Spray Ever
Warhammer Community · 13 June 2022
Backs Games Workshop stating in text, rather than in video, what separates its two most-confused ranges: Contrast paints are designed to tint surfaces and give an intense finish, while the reformulated Shades are designed to provide instant depth and stronger shadows and to settle into recesses while leaving raised parts relatively untouched. Also the 25 new Contrast colours, and White Scar spray described as designed specifically for use with Contrast paints.