Step-By-Step Weathering

There's no real way to explain any of this. it's VERY subjective and more than any other part of the painting process, the many ways, methods and techniques that you can employ are endless.
Most importantly, have a good idea of how a weathered plane looks, what goes into making it look as it does, and then figure out how you can paint it to look right.
This step-by-step is really simple, it's almost a hack job. But this isn't really a good subject, it's hard to be analytical with the organic This will just give you an idea of what I do.

STEP 1
I start off using the panel lines to make a selection. I make a new layer and fill it with a warm dark brown color. I zap it with the gaussian blur, set to where I think it looks right, and then it's ready.

This is one approach. Loammo likes to do the opposite, he surrounds his panel lines with a light color. Both methods are valid and more importantly they both look good

STEP 2
Next, using the panel lines again, I select the inverse, reduce it by quite a bit and feather it. I fill it with a very light yellow. White will work as well but any where you can add more color to your plane, the better.


After getting the layer mode and opacity they way I want, it looks like this-