This article is meant as a beginner's guide to painting Warhammer 40,000 models, and the focus will be on Space Marines but the techniques are applicable to all different races/armies for Warhammer 40,000.
It will handle the basics. One thing a beginner has to keep in mind is that painting a model like the 'Eavy Metal team from Games Workshop takes a lot of practice and patience. So don't expect your models to be perfect the first time you try (there are exceptions of course...). Also, remember that the steps and strategies outlined below are guidelines, not rules. If you are more comfortable painting a certain way that yields better results for you, then by all means stick with it. That being said, don't be afraid to experiment and try new things.
Why Paint Them?
Well obviously, they look a hell of a lot better painted (nothing is more boring than the dull plastic/metal they are made of). Secondly, if you want to game, especially on games days, it is mandatory to at least give your models a Basecoat.
Establishing a Hobby Area
You will want a place to keep all your sprues, bitz, paints, and models, such as a card table or a desk. You should have newspaper spread out and napkins handy for wiping your brush, and two glasses of water (one for washing, one for blending), preferably transparent, so you can easily tell when it needs to be changed. As for light, you should have a nice bright lamp, but direct sunlight is even better. It will help to have small plastic paint reservoirs handy for mixing colors, or you can even get empty Citadel paint tubs for storing mixed colors that you use often (handy for highlighting/shading entire armies with the same color scheme).
For models that are difficult to keep together with glue, you'll want a pin drill vice, some extra pin drill bits, paperclips, and Green Stuff, as well as a sculpting tool, especially if you're into conversions. Some of the newer metal models have these awful bits on the bottom of them that they use to remove them from their molds. Before you can glue them to their bases they must be removed, which can be extremely time-consuming unless you have a miniature hobby saw. You'll also want a basic hobby knife and a good place for cutting to get rid of other miscellaneous flak on models. All of the above materials are available from Games Workshop.
As for brushes, you will want to always have a fresh supply of fine detail brushes--you can never have enough of these. It's nice to keep a larger brush for undercoats or base colors. Remember to hang on to brushes that have lost their point, or are otherwise unusable for detail, because these are excellent for drybrushing, and you don't want to ruin a new brush with drybrushing. Always wash your brushes with soap and water when you finish painting. Never store them with the brush down, or they will lose their tip.
While completely optional, many hobbyists find it helpful to paint with a jeweller's magnifying visor. This is a pair of magnifying glasses on a visor that is strapped around your head, usually with a small lamp. They can be as cheap as US$20.
Step One: Assembling the Models
Before assembling you're models, it is recommended by many (including GW) to clean the models. When they come out of their mold, there might still be some residue of the chemical agents used to get them out of the molds. This residue can have negative effects on the way the paint stick/bonds to the plastic/metal.
To do is, just wash them with some water and soap, rinse them off good and let them dry. Once this is done, assemble your models.
Also, it's a good idea to wash your hands frequently, as in addition to the agents used to remove models from their molds, hand oil can interfere with the way your primer will bond to the figure.
Citadel's super glue is the way to go, generally. They do make glue specifically for plastic, since superglue leaves a weird white residue on plastic, but this problem is solved as soon as you apply primer.
For pieces that are difficult to keep together with just glue, drill a hole in both pieces so that they're lined up (with the pin drill it's surprisingly easy to get through the metal models, it just takes a little practice and estimation). Cut a short length of paperclip and apply glue to one end and insert it into one hole. Let it dry. Place the other piece into the proper place to make sure all is properly aligned and the paperclip is the right length. If that's okay, glue the other end of the paperclip and the surface of one of the pieces, and hold them together.
In order to cover up glue gaps or mold lines, you'll want to use Green Stuff.
It's all up to you to decide which colour scheme you want to use. Of course, if you are painting Ultramarines, it doesn't make sense to paint them pink ;) It all depends on the choice you make: are you building an army of an existing Chapter or are you building your own, home-made Chapter? Once you've decided on a colour scheme, the real work starts. First Undercoat your models as described in the next section.
To get better results when painting, undercoat your models. Depending on the colour scheme you want to use, you would want to use Chaos Black or Skull White as a undercoat. Both colours are also available in spraypaint canisters. This helps to speed things up.
For darker colour shemes, like Black Templars for example, use the Chaos Black undercoat. For lighter colour schemes like Blood Angels, use Skull White.
Once you've sprayed your models, check every nook and cranny to make sure you haven't missed any part. If needed, undercoat the missed parts with a slightly thinned down Chaos Black/Skull White.
Some painters will often choose to use a Chaos Black undercoat, and follow that with a heavy drybrush on the entire model of Skull White. With this method, when you paint on your base color, the recesses will be slightly darker, and the raised areas will be more vivid.
If you are having trouble with spray undercoats, and the spray comes out very grainy and obscures detail on the figure, remember to shake the can very, very well, especially if it's new. Also, remember that you want to hit the model mid-stream; bring the stream across the model, making sure to start spraying before you hit the model, and end the stream after you've passed it. This will ensure that you aren't getting any strange consistencies with the spray. It's also advisable not to keep using the can until it's absolutely empty, as the stuff near the end of its lifespan can come out strange.
If you still don't trust yourself with spray undercoats, or you prefer the control of a real brush, there are also paint-on primers available (it is advisable to use primer as opposed to a regular Chaos Black paint because of the way primers bond to metal/plastic). Unfortunately, Citadel no longer makes Smelly Primer, their paint-on primer. There are, however, a variety of hobby suppliers that carry paint-on acrylic primer.
This is fairly intuitive. Once you're satisfied with your dried undercoat, follow through with the color scheme you had planned. You may want to leave some of the recesses black if you undercoated in black, and you may want to not paint some of the details you want in a different color. If you were, for example, painting an Ultramarine, you would paint everything blue but leave the chest eagle, shoulder trim, eyes, mouthpiece, and bolter for later.
This is where your fine detail brushes come in. Here you paint the small areas that are important nevertheless, such as the shoulderpad trim or eyes. Faces are especially troublesome, but there are plenty of guides on the Internet and methods specifically for flesh, faces, and eyes. You may find you want to dilute the Citadel paints slightly.
Shading & Highlighting
Some old-school hobbyists disapprove of the 'Eavy Metal style of exaggerated highlights, but the rationale behind the method is that light falls differently on life-sized objects than they do on their miniatures, and the highlights are there in order to add more detail, to make it look like it's bigger than it actually is.
In general, one should start shading first. This involves looking for recesses, and painting them a darker shade of the base color you painted there before. If you undercoated black and painted your base color such that you left the recesses black, you don't have to do this if you're satisfied with your results. The easiest way to shade is to use an ink (which should be watered down even further), or dilute an acrylic paint. Be sure to wipe a little paint off your brush first so the ink doesn't flow everywhere. If things go as planned, the ink should sink into the recesses naturally. If you're having trouble with ink going where you don't want it to on large surfaces, try altering the consistency of your ink, or dabbing a small amount of rubbing alcohol on the area you don't want your ink to go to.
Highlighting is one of the trickiest methods to master. For hard edges, although time-consuming, the idea is quite simple. Paint a line across your edge of a slightly lighter shade than the base. It is helpful to wipe some paint off the brush first, and to use the side of the brush here. If you are happy with that, then decide where the most exposed edges are (imagine a "halo" of light above the model, or experiment with other light sources). Paint an even thinner highlight in the middle of the previous highlight on only the edges you think would be lit most.
For more continuous surfaces, it gets trickier. You'll want to prepare anywhere from 3 to 7 shades of your base color, starting from a shade very slightly lighter than your base color up to something much paler (note that if you are doing a small, detailed area, you can also do this in the opposite order, starting from white and working to a basic shade). As an example, let's suppose that you are painting a robe, and you have painted the base color as a nice, dark red. You'd take your slightly brighter shade of red, water it down plenty, wipe off the excess, and paint almost all the areas except the dark red recesses. After it has dried, you'd take an even more slightly brighter shade, water it, wipe it, and paint smaller areas, inside the previous areas. Repeat until you have gotten to your brightest shade. The key to this blending is that the paints are watered down nicely. To blend it even better, between steps take a shade between the layer you just painted and the previous layer, water it down, wipe it, and paint in tiny little circles along the border between the two colors. Be patient with this method, it takes plenty of practice, but once mastered is very effective.
Drybrushing is the easiest type of highlight. It involves starting with a darker shade on your model, getting a brighter shade on your brush, wiping off a lot of it and lightly brushing the intended surface. Because the paint has already been dried so much, it will stay out of the recesses because the brush won't reach them. Metallic details are usually good places to drybrush. Bone is also a great example. If you are, for instance, painting a Chaos Sorcerer with a giant skull design taking up an entire shoulderpad, I would start with a black undercoat, do a heavy drybrush of either Bestial Brown or Snakebite Leather, do a slightly lighter drybrush of Bleached Bone, then finish off with a sparse highlight in Skull White. It's a very simple method that yields astonishing results.
If you're very confident and ambitious, you can try what's called Non-Metallic Metals, or NMM. As the name suggests, this means painting metal areas using non-metallic paints. For example, you would paint a silver sword using only black, white, and shades of grey or blue. The advantage of NMM over metallics is that you have a greater degree of control over the way the lighting is reflected in highlights. It is extremely difficult to highlight metallics convincingly because non-metallic paints don't mix too well with metallic ones. There are more detailed guides on the Internet.
If you've just painted something that you know you want to keep the way it is for ever and ever, you should preserve it using varnish. Try and find clear, non-glossy varnish to preserve your work. You may also want to experiment with glossy varnish for a wet or shiny look.
You'll often find that you have plenty of models sitting around whose paint jobs seemed like a good idea at the time, but you really wish you could repaint them. As Citadel paints are acrylic, you could certainly just put another coat on, but you lose a lot of detail this way. There is a non-toxic bio-degradable cleaning product available in most stores in the US called Simple Green. Fill a shot glass up with the product, put your model in (make sure all of it is submerged; you may want to cover your container), and let it sit there for 24 hours. Use a toothbrush to get off any remaining paint flakes. The best part is that you can reuse the same container of Simple Green.
Green Stuff comes packaged as a double strip of yellow and blue putty. Use a hobby knife to cut off the desired amount, roll them together until it's a consistent green, and mold it to your liking. It's useful for filling gaps or hiding mold lines. Remember to keep your hands, the Green Stuff, and your tools wet, it makes it a lot easier to mold. Once it has dried, it's also possible to carve it out with your knife, handy for creating icons or engravings. For ideas and techniques on conversions with Green Stuff, check the GW website.
- In general paints right out of the tub are a bit too thick for anything but drybrushing. Normally you want to dilute it a little so that it is close to the consistency of milk.
- When painting start with darker colours first and later highlight with the lighter colours.
- Leave the deeper parts of the model dark or even black to suggest shadows.
- Bright colours and white have relatively weak pigments. Use white (when painting white on a dark colour paint 2-4 layers) or "Fortress Grey" as a basecoat before applying the bright colours.
- Use multiple layers of watered down paint opposed to one thick layer. This will enhance the quality of the paint work and will avoid obscuring details on the model.
Different step-by-step guides can also be found on the Games Workshop website. Check out the different races/armies, most of them (if not all), have a guide on painting them in the "getting-started" section.
An overview can also be found which lists all articles related to painting GW models: Painting articles from GW.
For a very in-depth guide on miniature painting in general, check out this FAQ: 
To see some amazing paintings and conversions, go to CoolMiniOrNot: