Priming is something that most mini painters hate. It is a chore and a massive drain on time and energy to do on a large scale. Since the advent of reasonably durable spray varnishes, I have even gone through periods where I didn't prime at all (it is not as bad an idea as you think but not something I do any more especially on highly detailed figures). There are any number of people that will tell you how to do it their way. I am more about the pros and cons.
THE TECHNIQUES
The Brush
Traditionally (well maybe historically) priming is done with a brush. This has the advantage of ensuring that all the crevices are covered with paint. The main downfall, to my mind at least, is that it takes a lot of time. This is made up for by cheapness and the ability to do it any time of year. You are also doing it with a brush which rarely gives a fully even coat. I generally only do things this way when I have spare paint on the palette.
The Rattle Can
Rattle cans have been around for a long time and they became more of a thing during the nineties. It is much quicker than painting. You generally get a more even smoother, more even coat of paint to work on. Some brands of paint come colour matched between rattle can and acrylic paint. On the downside you generally have what you have in the house or have to buy it. It is not something you are going to do in the house or even close to the house. You really don't want to breath the stuff in so you need a well ventilated area or a face mask. Rattle can propellent is a nasty enduring smell and you really don't want to breath it in. Manipulating the figures to get the paint in the crevices is relatively hard work. Spraying in cold weather (anything below 15 centigrade or about 60 fahrenheit) is a really bad idea and the effects can be awful. Some types may require higher temperatures so always check the back of the can. Rattle cans are not cheap and some are cheaper than others but they are a lot quicker than brush priming. Spray primer has the side effect of helping to bind texture to the base and makes a nice consistent colour for painting the base later.
NB you can get some colours that are designed for plastic. If you look closely they are also good for non-ferrous metals which makes it a great choice all round. My experience is that acrylic and car primers in general are better. Enamel is also available and is often cheaper but slightly shiny.
The Airbrush
Airbrushes are a great piece of kit and you can get some effects that are really difficult to achieve with a brush. You can use them inside the house without gassing yourself and your family (I would still use ventilation and some sort of extraction system as you don't really need the particulates in your lungs so better wear a mask especially if your ventilation isn't up to standard). They are a bit easier to manipulate than rattle cans so getting paint in the crevices is a lot easier. It gives you the ability to prime in whatever colour you want. Airbrushes are not a cheap option. A half decent airbrush and compressor is going to cost at an absolute minimum about £200 or $250 ish and probably closer to £300 or $400 ish. Then you need paint, thinner cleaning fluid and any number of other bits. Then there is the cleaning and maintenance. You are not just going to use it for priming so it might be a good idea if you like using an airbrush. Like the rattle can this is a good way to bind texture to the base.
THE COLOURS
In general what you are looking for in a nice flat matt colour.
Black
I started by painting everything black. These days I only use black for certain figures that are going to benefit from it. Oddly, and don't ask me to explain, a base coat of black is not always the right choice for figures that are all black as you are really trying to paint something that is extremely dark gray to get a sense of texture. Miniatures that are generally very dark (bark brown, midnight blue) or are mostly silver, steel or iron, and anything that is black (Darth Vader). It might be more useful for figures that come in pieces and some vehicles. It has the advantage that if you miss a crevice when painting then it is clearly an attempt to make it shadow rather than the fact that you missed something out. In this day where talent is in liquid form (that's washes to you) this is not really an issue. The downside is that it make the paint on job of it very dark which makes painting brighter and less pigmented colours very difficult.
White
Is an obvious choice for very light coloured figures. It really holds inks and washes well. I sometimes concentrate on using washes alone. Using white as a base can give a really ethereal effect. White is a difficult colour to shade and make it look good. In order to get shading then it might be worth while spraying white over light grey. In my experience white works better on smaller scales than 28mm in order to lift the colours.
Grey
Light grey primers are where I go to as a default. Although they lack some of the things you can get from the extremes of black and white, it is the best choice for most colours as it is not too light or too dark and holds all colours well enough. It works especially well with brighter colours, they seem to have a depth of colour without the washed out colour that white can bring. Unlike much of the other stuff I talk about here there a few negatives I can come up with.
Coloured
Many hobby useful colours have been around for a long time but there has been a profusion of suitable colours in the last decade or so. The advantage is that the primer is also the right colour for the main paint scheme so you are saving yourself a lot of time. I use these mainly for units of figures that have the same basic colour normally uniforms, where the figure is predominantly naked, are covered in fur of have some overwhelming colour. This works for a lot of colours but they can undermine other colours painted on top of them especially if the base is light. This can be a major time saver.
