It's easy to forget mobile suits are war machines, especially when our model kits come out of the box pristine and shining. However, battlefield equipment doesn't stay unscathed for long, and so adding damage to your Gunpla (in addition to weathering) is an important way to create a more realistic appearance and make your models more unique. There's no limit on how extensive and elaborate you can make your damage, but the basics are simple. In keeping with my philosophy of using beginner's methods, this guide covers my simple technique for creating battle damage, using one of my well-worn Wolfgar Zakus as an example.
We begin with the part we want to damage. I usually paint before applying damage to my parts, but since we'll be adding color to the damage itself, you can do it before painting just as well. In this case, I want to make a gash in the armor…
… So naturally the first thing to do is just cut the gash. Carefully use your knife to carve out the damage in the shape you want. Make small cuts and shave away a little at a time. It's easier to cut away more than to put some back if you decide you've taken too much.
Be careful not to cut too deeply, unless you intend to make a hole in the armor. If you do, keep in mind what's on the other side. Whether or not you're cutting all the way through, be careful not to cut away places on the part that're important to connecting it with the rest of the model.
If you're satisfied with the damage as you first cut it out, you can leave it at that, but in this case I decided to follow up with a file. Often the knife leaves jagged edges, and a file is ideal for smoothing these out. It's also a good way to increase the size of the damaged area very gradually.
You can also use files instead of a knife to create damage. In particular, a round file like the one in figure 3 is ideal for making bullet holes or graze-marks.
Once you're done shaping the damaged area, paint it a different color to show the armor material. (Mobile suits are armored with metal or other materials, after all.) I use a silver Gundam Marker, but of course you can use whatever color you like, and a brush works well here.
The most important part of adding damage, aside from the damage itself, is making the model look like it's actually been damaged. If a mobile suit's received battle damage, it's probably also just got some wear and tear, so weathering is essential to make damage look like it belongs. Here I've used a dark weathering compound to create the impression of burn marks around the damage on the shoulder and upper arm, since these dings are meant to be the result of sustaining some machine-gun fire. This darkening also helps blend the damage in with the rest of the arm, which shows a fair bit of rust and desert grime.
Note figure 5 shows this component before topcoating, so you can see stray grains of weathering compound, and in general the weathering is heavier than it will ultimately appear.
To complement the dings on the arm, here I've filed some round damage from grazing shells on the shoulder shield. You'll also notice I placed a ding in the flat of the shield to hide a botched decal application.
Here we see the damage as it comes together. In combination with weathering, a little damage goes a long way to give the model depth and heighten its character.