Here's something to think about when next using a primer, firstly most people grab for a GREY primer, NOW WHY USE THIS COLOUR ?
Grey, is made from the two most violent colours in the colour spectrum, ie, black and white, so why use it at all ?
It takes a couple of top colour coats, to just hide this grey colour, it's OK for getting depth of paint onto something like a car, so it can be polished back to a high gloss, but why TRY TO DO THAT to a model ?
If your going to use a single colour as a primer on a model, then think of the most neutral and natural colour that is on the planet, SAND, don't believe me eh, just look around the room where your now sitting. How many different shades of sand, are right near you now ?
This stuff will cover the dreaded red plastic, with just one easy thin coat of primer and then you can even use a single thin white top gloss coat over the sand colour. By using a MATT "colour" as a primer, you only need that one thin gloss coat to cover, now the matt paint has tiny imperfactions in the surface that takes the gloss from it, these imperfections are an excellent base, for the single gloss top coat to adhere too.
DO NOT knock it, until you have actually tried it out, serious doubters have become instant converts to this idea, once they have tried it out.
If you want to use a top gloss coat of say a green colour, then use a single matt primer coat of a similiar green colour, this will produce a rich green colour, that on a model, looks just right. You can expand this colour choice of matt undercoats, to whatever colour top coat your going to use.
This sand colour I use is, TESTORS Matt Sand, followed by a top coat of whatever colour enamel paint I care to use, sometimes I might use Testors enamel, other times I will use Dulux paints, straight from the hardware store, it's cheaper and there's lots more for the buck, if you get my drift.
I only use the cheapest form of carrier with the enamel paints to, Mineral Turpentine, works wonderfully well and it's cheap.
If you want to use acrylics, then it works just as well, as what it does for enamels, the "only difference is", the acrylics require a polishing to get them to look correct, so avoid that $hite like the plague, if possible.