Please note: I wrote this article some time ago for another site that I used to own. It's not really up to TG standards as it has no images to illustrate it. This is an issue that I hope to rectify as and when I have time to take suitable photographs but in the meantime I offer it here in it's original text only format:
There are already a million documents out there on how to paint but no site about model making would be complete without one so here's ours:
First of all make sure you use good paint. Opinions vary but you shouldn't go far wrong with the name brands.
The next thing is to use decent brushes. Don't make the mistake of thinking that sable or other natural hair brushes must be better than nylon. If you're using acrylic paint, natural brushes tend to take a beating and the nylon ones that are made specially for acrylic work are best.
Before you begin painting you also need to make sure that the model is perfectly clean. Dust, oil from your skin and residues left over from the moulding process will all serve to spoil your finish. Metal parts can be cleaned with alcohol while good old warm soapy water is the best all rounder. Be sure to rinse off all the soap before you tackle the problem of drying it!
A quick rub with a towel obviously isn't the answer as it will probably result in the model acquiring more fluff than you just washed off. The best bet is to leave it to air dry (after shaking off as many droplets of water as you can) perhaps with the assistance of a hairdryer on low heat if you're in a hurry.
This raises an interesting issue that we may as well deal with right now at the preparation stage: not only does the model need to dry after being washed, but there are going to be plenty of other occasions during the painting process when it's drying or waiting for you to get back to it and there's absolutely no point in it standing around where dust is likely to settle on it. In other words: organise a place for it to 'hang around'. One possible solution might be to have a board on which it stands (while you're working on it) that can be covered with a polythene tent (to keep the dust off) and placed out of harms way on the top of a wardrobe or bookcase.
The first application of paint will usually be an all over coat of white paint. This performs a number of functions:
Firstly it makes everything the same colour. Chances are that the model will be made of a variety of materials each with it's own natural colour and few, if any, of them will bear any resemblance to what the model is really supposed to be made of. A coat of white primer evens everything out and gives as good base on which to apply other colours such that they give a true representation of the final colour.
Secondly, an overall coat of a light colour will show up any imperfections in the surface so that they can be filled and sanded and re-primed until you have a good surface on which to get down to the real business of painting.
The final reason for priming is that the different materials used in the model may well react differently to the paint. Most fillers for example will absorb the first coat of paint (which then seals them and prevents them from absorbing further coats). Other materials will absorb the paint while others will not. Metals for example will often be easier to paint with acrylics if they have had an initial coat of oil base (enamel) primer while vinyl will react with oil based paint and should be primed with a water based acrylic before you put any other paint on it.
By the time you move on from the priming stage your model should have a consistent coating of paint over the entire surface that will ensure that further coats of paint will behave the same way and result in the same colour finish regardless of the nature of the underlying material.
I've referred to this stage as the application of base coats but this assumes that there is more painting to follow and this may not be the case. For example if you are painting a vehicle and want it to appear in 'as new' condition you may well stop painting after the application of these 'base coats'. If however you want the vehicle to appear weathered or if you are painting a figure you will almost certainly move on to further stages of painting.
Thinking about the order in which you will paint things is the key here. A good rule of thumb is to paint light colours before dark colours as it's easier to cover a light coloured error with darker coloured paint than it is to do the opposite, however painting the hard to reach places first also makes a lot of sense.
A wash is a thin paint that washes into the nooks and crannies on the model (in addition to tinting the flat areas) and it can add a dramatic depth to miniatures.
The bad news is that after all that care you took to get a good finish at the previous stage, the application of washes is likely to make it look awful and it's only after the later dry brushing stage that the results really start to shine though. Getting used to the results of the washing stage and knowing when to stop is a matter of experience I'm afraid.
Prepare the wash by thinning the paint with something like 2 parts thinner to 1 part paint. Use a brush to 'flow' this onto the model. Hold model such that the surface that you're working on is horizontal so that you can control how the wash flows.
Let the wash dry and remember that you can always add a second or third coat of wash if the first was too light so err on the side of caution.
When trying to decide on a wash colour for the purpose of enhancing shadows, start by deciding if the base colour is 'warm', like red and yellow, or 'cool', like green and blue. Dark brown makes a good wash for warm colours while black does a good job on cooler colours. For cool colours, you can also use darker shades of the base colour. This usually does not work as well for warm colours.
Another use for washes is to simulate the effects of mud and other gunge which builds up in crevices and in this case you need to choose a colour that best matches your gunge of choice.
What washes do for the crevices, dry brushing does for the raised areas. It has two main purposes. The first is to create highlights to counteract the shadows and the second is to simulate wear and tear, as it's the raised areas that always suffer most.
The technique is fairly simple: take an old brush (because this is harsh treatment for a brush and if the brush you use isn't considered old, it soon will be) and dip it into the paint. Now wipe the brush across a piece of kitchen paper until only a faint line of paint comes off the brush. Now gently brush across an edge or high point of the model. This process is repeated to build up the highlights as required.
The trick is to dry the brush quite well on the paper. You can always add more paint to the model and only a very small amount of paint is needed to get the required effect.
For highlights it is best to use white paint or white tinted with a hint of the colour over which you will be dry brushing. For simulating wear you need to think in terms of what colour would show through. On a vehicle this may well be the silver colour of the metal. Bear in mind that damage in a non-wear intensive location will then tend to rust while in a high wear area the constant wear will keep the metal clean.
This final stage is optional. Basically the idea is to protect the models paint job from handling. Some like a coat of gloss varnish while a coat of matt will probably be more realistic in most cases and there are instances where both might be appropriate e.g. a matt figure wearing gloss boots.
Probably the best effect is obtained by omitting this stage altogether. Indeed with a model that is painted to simulate bare metal the chances of applying a coat of varnish without ruining the effect are slim. Obviously a model which had not been varnished is more prone to wear and tear but depending on what you do with your finished models this may not be an issue.