I think paint scratch and chip is one of the most overused technique in mecha modeling. It sounds really easy, you can simply dry brush some silver along the edges of armor and viola, nice and shiny. But if you overdo it, it actually looks like your worn out die cast toy.
I think it's more appropritate for finer scale kits such as 1/24 or 1/35, but not so suitable for 1/100. If you look at pro military modelers, the paint chips are sporadic on 1/35 tanks. Therefore with a huge mecha like 1/100, having a huge piece of silver from chipped paint is a little exaggerated - how was it created in the first place? If they're scratched off, it's likely that there are some collision and so the armor should be dent. So I usually associate paint chipping areas with battle damages on 1/100 kits. Nautral paint chipping and scratching without damages should not be very visible at all in 1/100, may be just a little around panel corners. Otherwise the scale of the mecha will be reduced.
Here Brutish Dog is 1/24, so I think it's sensible to add quite a bit of paint scratches. I use unthinned silver enamel and dry brush the silver starting from armor edges. The thick paint will usually end in a jagged edge, which looks a lot like paint worn off.
Another technique for doing this silvering is spray a base coat of silver, top with black and finally the actual color of the armor. Then you scratch off the paint and reveal the silver. This is supposed to create a very realistic paint scratch effect because afterall, this is how it happened. But I find it a bit tedious and not really necessary if you drybrush correctly.
Dust and Sand
Mecha walks and picks up dirt and sand on the ground. The stomping motion also send stuff on the ground flying, so some dirt will also fly up a little.
The color of the ground material depends on where the mecha is travelling. For example in cities, concrete will get picked up and so the material will be white or light grey in color. In swaps, mud gets picked up, and the material is dark brown (black + mars brown + little yellow ochre). In other places you get sand, and you get a tanish color (yellow ochre + white + little mars brown).
To create dust and sand, I mix the appropriate color with a slob of cold wax, which can create a very coarse grainy texture. Jab the mixture lightly onto the mech, make sure they disperse somewhat and doesn't look like a blob. Area near the sole will get most of it, but edges of the armor pieces surrounding the foot will also get some, like this Gouf.