I'm old enough to remember the old Airfix enamel paints in glass jars but I've been using acrylics for many years now. I brush paint using Vallejo, Lifecolor, Revell Aqua, Xtracrylics, and Games Workshop. No special primer. Any Humbrol acrylic spray can will do. Light grey or get a closer match to the final colour. A light coat, it etches into the plastic and won't scratch off. Dries in minutes and provides an excellent key for the top coats.
I have several different colour cans I just use for primer/undercoats. For example if it's WW2 RAF I'll spray Beige green undersides and then dark earth uppers, no masking, just a quick squirt top and bottom. Then brush paint the camouflage colours. Never had any trouble.