This is a run down of the current things I know about weathering mechas. My skills in this area is nowhere close to where I want to be, and is at best amateurish compared to other works out there. The excellent book by Shep Paine offers much more on the techniques and has a lot better writing.. afterall, I'm recently accused of writing broken English :)
I would also like to point out that no one has built a real giant robot yet, and therefore how these guys are weathered is very subjective. My take on weathering is of course based on my own reasoning, and may or may not be correct. Anyway enough disclaimers...
Weathering creates details and realism for your kits. Realism is about finding order to the chaos, which involves reasoning about how a certain effect happened and how to achieve that by painting. Without the order, the model becomes dirty rather than weathered. Many people also concentrate on realism but forget the detail part, and they apply the same kind of weathering technique on a 1/100 kit as they do on a 1/35 kit. As a result, the weathering visually reduces the scale of the kit. Examples are huge areas of silver for scratched paint and great blobs of rust. The difficult aspect of weathering is thus subtlety and knowing when to stop. The easiest mistake beginners make (including myself) is to keep on weathering, thinking each application of a technique will enhance the look of the kits, but instead go overboard and the kit just looks trashed instead of weathered (e.g. I think my Ez8 is a bit overdone, but the NT-1 and the Brutishdog are more reasonable). A good stopping rule is when you think you just need a few more strokes to get the look you want, stop. Experience helps a lot in this area.
Anyhow enough dry stuff. I weather my kits exclusively with artist oil. It's a medium not used by much of the mecha modeling community, but I like the control you can get from it, and redoing an area is extremely easy. It is also not difficult to create subtle effect with oil. In the sections that follows, I'll talk about working with oil first, then various effects and how they're achieved. I usually weather after decals are applied so that the decals will get weathered too.
|Working with artist oil|
|Armor highlights/Drybrushing & more|
|Rain marks, oil spills and rust|
|Paint scratch, Sand and dust|