I always prime a piece before painting. Most of the time I’ll use Testor’s Dullcote as an “invisible” primer. It creates a bit of texture, allowing the paint to “grab” on. Without it, water-based acrylics can react in unexpected ways. On occasion I’ll use a white or grey primer, if I want to cover the whole figure. But I try to avoid that as it’s just one more layer of thickness to deal with in joints.
The reason I always use primer has to do with the bonding agent in acrylic paint. Acrylics are water based, and most of the time I’m at least partially painting on top of plastic, which repels water. To see what I mean, grab a couple of scrap figures or parts. Spray one with Dullcote. Next, water down some paint until it’s fairly thin, more like a wash. Apply the watered-down paint to both test pieces. The paint on the primed piece should apply fairly evenly wherever you apply it. On the unprimed piece, the thin paint will likely pool into spots, like water on a clean glass. The primer allows for greater control over the application of the paint.
Obviously that’s an exaggerated effect. Thicker paint is not as prone to such pooling, but it does still come into play. I prefer to avoid it.
(And don’t forget to spray in a well ventilated area, and/or with a respirator of some kind.)