Hey leemitcheltree + badlanguage – Thanks for the nice words. Really appreciate it!
To be honest, I leaned pretty heavily on the steps outlined in Mig Jimenez's book, "In Combat: Painting Mechas".
But to break it down really briefly, I'd say:
1.) Prime with Mr. Surfacer 1500
2.) Mix Tamiya colors until you're happy w/ the color combination and lay down clean colors of the mech's base colors
3.) Preserve base coat w/ layer of Future
4.) Chip heavily w/ brown paint and silver paint (depending on the area of the mech), using tiny, tiny brush and trying to place chips in places where they'd occur naturally, ie, along protruding edges, near seams, places where it'd get banged up. Also try to not make them uniform all over, although I wish I'd have taken my own advice a bit more for that...
5.) Lay down some hairspray on places where you wanna do the streaking grime effect (i did it on the upper legs/crotch) and then spray some streaking grime from your airbrush on these spots and chip it off a bit (with a stiff brush and some water) to have a nice contrast in weathering from all the chips you painted all over the place
6.) Do some engine grime, streaking grime, wherever it looks natural to have the dripping down, grimey effect. Do this by painting it near chips or seams or whever and then wiping it downward w/ some spirits and a clean brush to soften the edge and make it look like it's running downward w/ gravity. I probably should have done this a bit more, you can see it a bit on the yellow squares near his crotch – The drips beneath them. I'm not the best at this so tend to not do it too much.
7.) Throw some rust/dirt colored dust pigment in joints or near the feet where dirt/dust would accumulate at the end for another contrast in weathering technique to the chips and grime.
The bulk of the work on this model came from getting the base colors looking the way I wanted them to and all the masking (w/ tamiya tape) that that required. The model came in colored plastic already, but the colors were a bit gross so it was a big decision to commit to all the masking. Also had to wipe parts back to bare plastic a few times due to poor masking. Bleh. But once I was happy w/ the base colors, I tried to lock them in w/ a solid coat of future which made the model pretty durable for the weathering ahead. The next most time-consuming part came from painting all the chips w/ the tiniest brush. This was easily a few solid days. Yikes. And from there, the engine grime and pigments were pretty quick little touches at the end the bring contrast the all the chips (which in my case, were a bit too monotonous and uniform, allowing the grime and pigments to hopefully lend some contrast).
Hope that helps! Let me know if you have any questions. And totally check out Mig's book because I was really just pulling from his techniques all along.
Cheers and thanks again!