Zenithal priming borrows a word from astronomy: the zenith is the point directly overhead, and a zenithal prime is one sprayed from above so that the model ends up lit the way an object outdoors is lit. It is done before any colour goes on, and it means the finished model already has light and shadow built into it rather than painted onto it.
The two or three passes
Start with a dark primer over the whole model, from every angle, so nothing is left bare. Then spray a mid or light tone from perhaps forty five degrees above, which catches the upper surfaces and leaves the undersides dark. A third pass of white from directly overhead, used sparingly, sharpens the brightest points. The gradient this creates between the three is the whole product: upward faces bright, downward faces dark, and a soft transition between them that no brush technique produces as quickly.
Why contrast paints depend on it
Contrast and speed paints are translucent, so whatever is underneath shows through. Applied over a flat undercoat they give a flat result with shading only where the paint pools. Applied over a zenithal prime they inherit the gradient, and the model reads as lit and shaded from a single coat of colour. This combination is the reason zenithal priming stopped being a display-painter's technique and became a mainstream one: it is the fastest route from bare plastic to a model that looks finished.
Where a zenithal highlight goes wrong
The usual errors are too much white and too steep an angle. Heavy white leaves a chalky, washed-out model that colour struggles to recover, and spraying from too directly overhead creates a hard line rather than a gradient. Zenithal highlighting is meant to suggest light, not to draw it. The other common problem is applying it and then covering it with opaque base coats, which wastes the effort entirely; the technique only pays off under translucent paint, glazes or thin coats.
Questions people ask about zenithal
What is zenithal priming?
Priming dark overall, then spraying a lighter tone from above so the model carries light and shadow before any colour is applied.
Do I need an airbrush for a zenithal highlight?
It is far easier with one, because the soft gradient is the point. Spray cans can do it at a rougher scale by spraying from a consistent angle.
Why did my zenithal prime disappear?
Almost certainly opaque base coats over it. The technique only shows through translucent paints, glazes or genuinely thin coats.