First I wanted to make it known, there is no proper or improper way to do this technique. This is just the way I did the chipping on my geara doga. The biggest thing with weathering of any sort is scale, if you can keep your weathering; or in this case, chipping in scale. It should remain somewhat believable. Secondly location and the amount of weathering is also important. Weathering is something that is incredibly easy to over-do and under-do. Just try to find the sweet spot.
Another thing that I'm unsure of is how this will format, I'm posting this from my phone. Now. On to the tutorial.
The required materials for this technique are, a fine brush (or a few, if you prefer) your paints (I chose colors based on the paints that I used on the frame. And some contrasting colors) and of course your piece that's ready for chipping. It's best to have it painted, decaled and gloss coated before you begin this process. Otherwise you'll be creating more work for yourself later.
Now the way that I'm going to cover this is by gradually building the chipping based on levels of usage and damage.
The minimum here would be completely unweathered. Or perhaps incredibly light chipping around major joints such as the knees and elbows. Which is where I'm going to start. Since we're using the arm as an example. Investigate the piece and look at what parts will move first, and what will rub against each other. In this example, below the shoulder, a small spot around the elbow joint and around the wrist cuff were areas I thought would possibly make contact with one another.
So the first thing you'll need to do is get your paint ready. Since I'm using Vallejo, I don't want to thin it. I find it gives me the best results. Next, you don't need very much. So I just apply a drop to my tray. Next I alternate between dotting and very gently and finely 'lining' the hard edges.
The technique I use here is to trace the line with your eyes a couple of times first. Try to stay as close to contact points as possible if you want light weather. One thing I like to do is draw fine short lines and leave large gaps between them. Then once you're satisfied, go back and dot in between and along edges, as you see fit.
Now, if you want to increase wear, expand your chipping to more edges. And possibly even along panel lines. Since I'm going for a more heavily weathered look, I went on all the edges.
Now to finish it off, I wanted to add some chipping inside of the opened areas of the arm. So I used the same method as the edging, but I tried rotating the part and changing the direction of my chipping. As well as rotating the brush, and changing my pattern, as well as brushing in various sizes, to make the chips appear more realistic.
Now, once you've finished chipping and you're happy with what you've done, (if you're not you should easily be able to wipe them away with some water and a q tip.) I like to go in and add a little depth to some of my larger chips. For this I use a high Contrast color and carefully put it inside some of the larger chips. (or all if you so desire) the change should be pretty drastic, just try to leave some of the dark edges from the original chip visible.
Finally once that's finished. I go in once more using another dark color. Something in between the two original colors. And carefully dot it in the contrasted chips. As well as a few of the ones I may have missed, or even some small chips you hadn't touched before. The key is to experiment and see what sort of results you can get.
Again this whole thing is something I'm still incredibly new at, just experiment until you're happy. And if you're not, and you gloss coated properly, anything you do should be easy to undo.