weathering thoughts
Mud: how think and how messy?
My weathering kit for vehicles includes weathering powders (available from Micro Mark), an eyeshadow sponge to apply it with, and my own concoction of inks and chalk pastels applied with a toothbrush or rough chisel brush
What I use and how I apply it depends on how wet I want the mud to look and how messy i want it to get. Dry mud/dust/general grime/rust is accomplished well with simple weathering powders. Load up your small round make-up sponge on a stick with the desired color and carefully blot it onto the areas you want weathered. I typically start with a darker powder and work up to a lighter one, sealing each successive layer with a light dusting of spray matte coat.
For thick fresh grime I make myself a simple potion from artists inks and crushed cheapo craft store chalk pastels. I pick a base tone ink like sepia or chestnut for rust and add grind up two colors of pastel to add for thickness, grit, and color. My favorite rust concoction is sepia ink with equal parts orange and black pastel mixed to the consistency of runny outmeal. Different thicknesses yeild differnt results. The thicker the mix, the chunkier yer mud. Then I load up a small stiff bristle toothbrush and riffle my thumbnail across it. This produces a nicely random spray of glop that yields realistic mud results for me. Again you start with a darker mix than desired and when dry you can highlight it with regular painting techniques in more precise colors. Or you can flick successive layers of lighter pastel/ink mix to build up a hefty layer of crust. Remember that wet mud is darker than dry mud and you will have both kinds of mud on your tank, typically more dry mud the higher up on the vehicle you go.
When done with the crusty splash I still apply a simple light weathering powder coat to the upper dried mud levels to make the transitions between clean and dirty vehicle a bit softer.
Hope that helps somewhat.
--number9