Primer or flat paint?

[Valvefloat]

What is the difference between a primer and flat paint?

[peteski]

Primer (especially etching primer) is a special coating designed to improve paint coat's adhesion and also is very opaque to provide a neutral evenly colored surface for the paint.  Sandable primers have more body to them to allow some light sanding for smoothing out any small uneven spots.  Actually primers are usually nowhere as "flat" as many flat paints.  They often produce an eggshell (sort of satin) type of surface.  If you really want to know more details see https://en.wikipedia.org/wiki/Primer_(paint)

Flat paint is any glossy paint with flattenign agent added to it to make it not glossy. 

[Dave G.]

Flat paint is just a coat of paint. Primer has the properties to act as a medium between the plastic ( in our case) and the paint. IE, primer sticks to plastic, paint sticks to primer. Not all paints, flat or otherwise, really stick well to plastic. Some paints are horrible at it and flake off over time, or crack, chip etc. In many/most cases, The same paint put down on a correct primer coat display no such lack of durability.

Primers made for kit plastic display exceptional bond and some degree of flex. Some are sealer rated so that colored plastics don't bleed through to light colored paints. Some build for smoothing and sanding etc.