Weathering is a sometime over looked part of painting by a lot of gamers. Adding a couple easy steps to a model can really add to the realism and interest of your army. Over the next couple weeks I'll be discussing some simple techniques that will be worth your time in the long run.
You can use the sponge technique for a couple of different things. It can be used to simulate dirt and grime or be used to look as if the paint is chipping off the surface of your tank, armor, or vehicle and exposing the primer underneath.
To execute this step take a sponge and the paint color you would like to apply. Dip the sponge in the paint and use a paper plate or a napkin/paper towel to dab most of the paint off. Then you simply dab on the paint to the area you would like the weathering or damage. I've found the places this looks the best is when applied to the edges or lower portions of things as this is most likely to get damaged or rubbed against things. Things like the steps on ladders and dozer blades are other examples of things that would have heavy damage to the paint. I use to different sponges for my sponge weathering. A fine sponge which can be found from foam in the back of blister packs. Also a coarse sponge that I simply use a sea sponge for.
Here are the sponges I use.
This first example of sponge weathering is on my land raider. "porn raider"
Notice How I applied the weathering over the murals. This adds realism to the piece as a whole. It breaks up the mural and makes it look like the tank has seen active duty since the servitor painted it on there. I used GW scorched brown on this with a coarse sponge.
This example is a Gundam model kit I had laying around. Most mecha cartoons the robots are depicted nice clean and pristine. I being a 40k player and fan wanted it to reflect a more realistic appearance of being a giant war machine. Thus I added weathering. The first weathering I applied was the sponge technique with a fine sponge using Vallejo German C. Black.
My last example is a necron monolith. I used a combination of GW scorched brown and Vallejo German C. Black. I've found the best colors to use are darker neutral tones. I tried sticking to the edges and the lower sections are done more heavily than the upper ones.