Plastic model primer exists because injection-moulded plastic is a poor surface for paint: it is smooth, slightly waxy from the release agent used in the mould, and chemically uninterested in bonding with an acrylic film. Primer solves all three at once, and skipping it is the reason for most paint that chips, lifts or rubs away on models that were otherwise painted carefully.
What mould release does
Injection moulding uses a release agent so the part leaves the tool cleanly, and a trace of it remains on the surface. It is invisible, it repels water-based paint in patches, and it is the reason a primer coat sometimes beads or dries unevenly on one area of an otherwise fine model. Washing the parts in warm water with a little washing-up liquid and letting them dry fully removes it. On plastic this is a good habit; on resin, where the release agent is used far more heavily, it is not optional.
Priming on the sprue or after assembly
Priming on the sprue is fast and gets primer into places that are hard to reach later, and it has one real drawback: the cut points where parts leave the sprue will be bare plastic, and glue does not bond well to primed surfaces. Priming after assembly avoids both problems and makes some interior surfaces unreachable. The common compromise is to assemble subassemblies, prime those separately, and keep glue surfaces masked or scraped clean. Which approach suits depends far more on the kit than on any general rule.
Thin coats, and the detail you paid for
The temptation with primer on a large kit is to cover it quickly, and a heavy coat is exactly what destroys panel lines and rivets, which is most of what a scale kit is sold on. Two light passes from different angles cover better than one heavy one and leave the surface detail intact. The same rule that applies to a 28mm figure applies to a large kit, and the consequences of ignoring it are more visible, because a filled panel line on a flat surface is obvious in a way that a filled fold in a cloak is not.
Questions people ask about plastic model primer
Do plastic models need primer?
Yes. Bare injection-moulded plastic is smooth and carries traces of mould release, so paint applied directly lifts at every handled edge.
Should I wash a plastic kit before priming?
It is a good habit on plastic and essential on resin. Warm water and a little washing-up liquid removes the mould release that makes primer bead.
Can I prime on the sprue?
You can, and it is fast, but the cut points will be bare and glue bonds poorly to primed plastic. Many modellers prime subassemblies instead.