Priming is the first step of painting, so it’s only appropriate that we’re finally talking about it on, what is this, day nineteen? Oh well, better late than never.
Primer, also called the undercoat, is the first layer of paint applied to your model. It’s used as a matte, non-reflective surface with “teeth” that makes it easier for your further layers to adhere. When I first started painting years ago, I couldn't figure out why my paint wouldn't stick. I didn't know I needed primer (and I was painting metal minis, which was even worse).
Primer is usually a flat, neutral colour - black or dark grey is often recommended for beginners, because it hides imperfections. Or at least makes your missed spots look like shadows. There are plenty of reasons to use other colour primers, too. With contrast paints, like The Army Painter's Speedpaint, you really want to use a white primer to make the most of the built-in shadows and highlights. For my zombies that I've shown a few times, I used a yellowish Bone White primer to give a more sickly base and overall grungier look to my undead figures.
A popular technique for priming when using Speedpaint is called "Slapchop," which involves putting down a black primer, then a coating of lighter grey primer, topped off with a drybrush of white on just the highest points. This helps enhance the effectiveness of your contrast paints, to make the highlights brighter and the lowlights darker.
Primer can be applied in numerous ways. A spray rattle can is a popular choice, and brands such as Vallejo and The Army Painter make a variety of colours. You can also use the spray cans you buy at the hardware store, but the quality varies tremendously. I love the Rustoleum Grey primer, but I can't find a decent white primer to save my life (white, in general, is a tough colour to get in a consistent quality, in any kind of paint).
Those who use an airbrush swear by its use for priming, which makes perfect sense. It combines the smooth, even coats of aerosol paints but without the harsh smells and mess of rattle cans. In a pinch, you can also use brush-on primer. It may not be as smooth as aerosol paints, but for those who don't have an airbrush and either can't or don't want to use rattle cans, brush-on primer is a godsend.
I used a lot of brush-on primer during the winter because the weather was too wet and cold for spray paint - I don't have a fume hood so I have do do my spray-painting outside. This brings us to a final important part of priming - the right conditions. To get your best primer coat, you want your model to be as clean as possible, which will involve clipping and sanding rough areas, then giving it a good wash with soap and water, and then letting it completely dry. If you're doing any kind of spray primer, you also need the right atmospheric conditions - not too hot, not too cold, not too humid, not too dry. Spray painting outdoors in the winter often leaves you with lumpy, runny primer, which will totally ruin your paint job even before you start.