As a newcomer to miniature painting, I'm, er, fortunate to have made lots of stupid mistakes early in my hobby journey. In my own defence, I did a lot of research before starting to paint. That saved me from some really egregious errors. But I managed to make a mess of a few things. Here's what they are, and how to avoid them, should you start painting miniatures:
Before you paint a plastic miniature, you have to "prime" it with an undercoat. I don't understand the chemistry of it, but an undercoat paint adheres to plastic better than other paints. This means the decorative paints you apply later are able to adhere to the undercoat instead of rubbing off of the bare plastic.
Beyond the mysteries of chemistry, the undercoat step is a little confusing because some plastic models come pre-primed while others require priming. And in both cases, it seems like it's just a footnote in the instructions. It seems to me that the first step for any model ought to tell you whether you do or do not need to prime before painting.
When you're unsure, give your miniature an undercoat. In my experience, a spray can undercoat paint is the best option. When I first started painting, I brushed on an undercoat because I couldn't find spray can undercoat paints anywhere. I ended up purchasing a primer paint in a drop bottle. It worked alright, but it was tricky to get right and it often either left gaps or it gathered in recesses and covered up the little details of the underlying sculpt.
Buy a primer or undercoat paint in a spray can. It'll provide you with a light and smooth undercoating with uniform coverage.
One point of confusion for me was that everyone seemed to use the term "primer" like it was a special kind of paint. I asked around at the local hardware stores, and while some people seemed to acknowledge the concept of paint that was a "primer", nobody could ever find a spray paint labelled "primer". Then I remembered something that had been true so far, and remains to be true to this day: When in doubt, just buy Citadel.
Sure enough, I went to the Warhammer shop an hour away and asked Jamie for primer. He explained to me that it's called an "undercoat" by Citadel, and of course they sell spray paint ("rattle cans") in any variety of colour. I use White Scar largely because I paint almost exclusively with Contrast paint, but I could see myself choosing a different colour for, say, a Space Marine that's 95% Blue or Red or Black.
I've since discovered that "primer" is a generic term for the act of laying down a coat of paint that you then intend to paint on top of. As long as the spray paint you buy specifically says that it works on plastic and is as matte as you can find, then you can actually (technically) use any old brand of spray paint. I've used Rust-oleum, Colour Lab, Army Painter, and of course Citadel. I admit that I prefer Army Painter and Citadel. I think they must be specially formulated to be even more matte than the "ultra matte" versions of general purpose spray paints, but I've primed lots of miniatures with general purpose spray paint and it works fine. It's a matter of availability, for me. I prefer the miniature paint, but I'm comfortable with general purpose paint in a pinch.
The undercoat does make a difference. If you're painting a squad that you want to look basically the same, then use the same undercoat for each model. If you change the undercoat from white to black or even just to a bone white, you'll be able to tell even after you've painted the miniature (a model with a white undercoat tends to have more vibrant colours than a model with a black undercoat).
Painting a tiny plastic miniature can be daunting. The more you pay for that toy soldier, the scarier it is to "ruin" it with a mistake.
Don't worry. It's embarrassingly simple to correct paint mistakes. There are two techniques.
If you've just applied paint to the wrong place, and the paint hasn't dried yet, just clean your brush, load it up with some water, and wash the errant paint away.
If you've applied paint to the wrong place but you didn't notice it until it was dry, you can paint over it. The fail-safe trick is to paint over your mistake with the same colour as the undercoat. Wait for that to dry, and then paint over the "patch" with whatever base paint was meant to be there. This is useful for Citadel Contrast paint, which tends to be semi-transparent. If you're using a particularly opaque paint, though, you might get away with just painting the base paint on top of your mistake.
This seems really obvious to me now, but I probably painted 20 miniatures before learning this trick. If you're not an experienced painter, it can be easy to fall for the illusion of what you're painting. The only thing that counts is what you see in the end. If you accidentally spill some blue "armour" onto a brown "leather" holster, you haven't transmuted the material from leather to ceramite, you've just spilled some paint. It's not really leather, and it's not really ceramite. Take the blue spillage back to the undercoating, re-apply brown, and it's "leather" again!
There's no painting tutorial on the entire Internet that says it's OK to apply paint (aside from Citadel Contrast and Vallejo Xpress) straight from the paint pot to your miniature. Lots of miniature paints are made with the expectation that the painter is going to use them along with small quantities of water. I guess providing them thicker than required allows for flexibility. But that means it's up to you to take a little bit of paint from the pot, put it onto a palette, and then dab in just a brushful of water.
Apply one thin coat, let it dry, and then apply a second thin coat. It must be mere fraction of a millimeter difference, but you can actually tell the difference between a mini that's been painted with thin paints and one that's been painted straight from the pot.
For the most part, none of this applies when you use Citadel Contrast or Xpress Color. They're generally designed and delivered thin.
If you're really just starting, do yourself a favour and pick up some cheap miniatures. I raided my board game collection and painted every miniature playing piece I could find. Had I thought about it, I might have taken it a step further and hunted down some cheap plastic toys (dinosaurs, army men, farm animals, and so on) to paint. To me, it just makes sense to have a cheap test subject when you start painting.
A plastic miniature is never technically beyond help. You can always repaint. But it's not necessarily easy to repaint, so I think it's worth for your first few attempts to be miniatures you're unlikely to ever want to play with. Give it an undercoat, feel out the base and layer paints, try dry-brushing, try highlight edges. Let these be the worst of your attempts, and then graduate to miniatures you intend to take to your tabletop games.
I think there's a reason Games Workshop promotes Space Marines as much as they do, and I don't actually think it's because they're cool superhuman war machines. I think Space Marines are the figurehead of Warhammer 40,000 because they're dead simple to paint. Really. Give it an undercoat, paint on some metal bits, highlight the pauldrons, and you're done. It's the bare minimum paint job I can imagine for a painted army, and they look amazing.
There's a lesson here.
Once you've graduated yourself from the cheap test miniatures you used to learn how paint works, grab a box of models that only call for 3 or 4 paints. They don't have to be Games Workshop Space Marines, but find something based around a similar work ethic: 80% undercoat and 20% decoration. This gets you on to "real" miniatures but it doesn't overwhelm you. You'll get a handful of toy soldiers you're proud of and happy to use in your game, and it'll give you the confidence to make your next box something more complex.
I'm pleased to say that while I've learned a lot over the past year, I'm still making lots of mistakes. Maybe a sequel post is in my future. For now, though, I've documented my errors here and hopefully it'll help a new hobbyist avoid the same mistakes, or maybe it reminds an experienced hobbyist of what it's like to be a beginner again.
Now get off the computer and go paint!
T'au soldiers photo by Seth Kenlon. Creative Commons cc0.