Many people seem to see weathering of miniatures and models as some arcane science practiced only by those with a lot of skill, or a lot of time. I'm here to say this isn't the case. Weathering can be a very simple process, but you need to know some of how to go about it. There are hundreds of techniques covered by as many or more blogs about weathering. Many go on about weathering powders, which I personally have never used. Others use salt, hair spray, or other techniques. All of those techniques have their place, and can be very useful.
I use 3 different techniques, which I will document further on Thursday with pics, but today I'll be going over some of the techniques without visuals to give you the idea of how to do it without pushing you towards using my exact technique (since everyone finds their own way of doing things that fits their painting style).
My drop pod posts have discussed one method I find is very good for large area weathering, but it also works for small areas (like shoulderpads, legs, etc). This is the use of masking fluid over a base weathered area.
For this technique you will need a few things, which you likely have most of already. First a couple paint brushes (on should be rather worn out), the worn out one being your 'stippling' brush. A base color for the weathering, and 2 colors used for 'rust'. In this case I find a dark color like black or a charcoal grey works great as the base coat for the weathering. Then use of browns in two shades for rust, in this case I used GW Scortched Brown, and P3 Bootstrap Leather. An alternative to the P3 is GW Snakebite Leather, or using Scortched Brown (3 drops) + 2 drops yellow + 1 drop white + 5 drops water to make a more yellowed uppper color for the rust.
You will also need a sponge and masking fluid. The sponge is easy to get, open a blister pack for nearly any miniature and you have enough sponge for a number of weathering uses. I use Winsor & Newton Art Masking Fluid, which is a bit expensive locally (Micheal's $13.99). This works VERY well, gives you enough for hundreds of miniatures and dozens of models, and dries quickly (within a minute).
Prime you model or miniature!!!! Let me repeat, Prime your miniature!!!!
This step is by far the most important for ANY painting. Primer has special binders in it that help it to 'grab' onto the surface, which gives a much more stable and durable surface to paint on. If you don't prime, don't complain when the paint flakes off, rubs off immediate, or acts very oddly while painting.
Next you give a nice thin coat of your weathering base coat, I find that thinning down your paint to the consistency of milk and working on several layers to cover is the best way to work in general. It takes longer but gives a much nicer look and feel to the miniatures and models.
Once that is done, get out your stippling brush and the darker color you will be using to make the 'rust' effect. Don't be shy at this point with it, and don't worry if you seem to have to much on the area you work on. Just dip, brush off until it's getting closer to dry brushing levels on the brush, and start quickly pushing the brush onto the surface. Wiggle it, smash it in, whatever you like. Just remember you can go back over with that stippling brush using the base color and 'fix' any overdone areas.
Once that is done, let it dry for a bit, say 5-10 minutes. This lets it set up more, and bind into the base layer. Never hurts to give a bit between layers, since it also gives you a chance to look over what you've done and see if you need to fix any errors.
Take the lighter color, and do as for the first color layer. Just have fun, but do it a bit lighter than the first color layer. This layer is the 'outer' layer of rust, which tends to be more exposed and lighter in color than the deeper rust. If you hit an area 4 times with the darker color, then 1 or 2 times with the lighter is just right.
Let it dry for a bit as before, then get out the sponge and masking fluid. I recommend cutting your blister pack sponge into long strips. Since that gives you a small squarish end to dip and use and the longer area to hold onto. Don't worry about the fluid getting on your hands, it peels off in less than a minute into a rubbery bit of garbage.
Dip the sponge into the fluid bottle, dab it on a piece of paper, paper towel, or something. Then just dap it onto the weathered area you've been working on. Let the texture of the sponge help keep it irregular. And feel free to let it be a bit heavily applied. Any area covered by the fluid will be your ending area of weathering (in this case rust).
Go ahead and paint up the model as you normally would, paint over the rough masking fluid area too (it won't hurt it at all). And once you are happy with your work, take some blue tac (or even an old tooth brush) and dab it and rub it along the areas covered in the masking fluid. It will come off in rubbery balls and strings, and the paint over it will flake away.
In the end you get a very convincing effect (such as in my newest drop pod post).
The next weathering effect I'd like to give some tips for is oil/chemical/dirt stains. If you look at pictures of old WW2 tanks you'll see discolored areas which are obviously made from chemical spills, or oil running from different fittings. This is by far the easiest weathering effect to do, and with Imperial Tanks for 40k it is VERY fitting.
You'll need 3 things, which I guarantee you have. A fair sized brush, a chosen color of the staining, and water.
Take your chosen color and on a palette add water until the paint becomes very very very very thin. Something along the lines of 1 brush of paint to 10 of water. You now have a seriously thin pigment wash. Take your brush, load it up, DO NOT BRUSH OUT ANYTHING. Then apply it with an upward brush stroke to the bottom of the fittings, edges, or areas you wish to have the stain coming from. Let the edge of the object pull the wash out of your brush, and let it run down the side of the model. This will give a very natural stain/drip effect.
Let it dry, reapply if it isn't heavy enough for your tastes.
Pretty simple really, but it helps make a model stand out, and making it feel used and not off the 'factory floor'.
Another easy to do weathering effect comes from a mistake I made and learned to like. I won't go into details on the mistake, but the quick and easy on it is 'prime from too close'.
Most primers say to use them from 8-12 inches away. Which tends to work pretty well. But I have found that if you do it from say... 4 inches away... at times you get the effect I like to call 'cracked looking area'. This can be pretty sweet, but I don't necessarily recommend it for everyone. It's not a guarantee that closer priming range will give it to you, and you definitely have ZERO control on where it shows up. But if you try it, you can end up with some sweet cracking effects that give the feeling of age to a model, or unrepaired damage.
Comments, critiques, arguements, helpful hints, feel free to reply to your hearts content!