This post is a little long because it's a mini-tutorial on a weathering technique I wanted to try, and it came out pretty successful, so I wanted to share.
If you look at the Star Wars studio models, they all have a grimey speckley texture on them. Word has it this was done by flicking paint off a toothbrush onto the model to create the speckles. However, this model is 1/72nd scale, and I was afraid the globs that flew off the toothbrush technique would be too large and hard to control.
So I came up with this idea and I tried it and it worked great. It gives you really tiny grime/speckles, and it's very easy to control to give you the exact effect you want.
I bought an air conditioner filter for 50 cents at Walmart. This material is very thin and has lots of small open texture to shoot an airbrush through. Basically using it as a stencil to shoot a grime texture.
After the wings were primed with white, I laid this material over the dried wings, and shot Flat Black through the foam filter material. You have to hold the airbrush right up to the foam almost touching it. Surprisingly little paint goes through, so you can build up the effect gradually without worrying about spraying too much at once.
After doing a few passes on the wings, you end up with this. A really nice small scale grime speckle texture...
Obviously it is too harsh in some places, so I mixed up the base color for the X-wing hull, Tamiya Flat White with a few drops of Tamiya Sky Gray to darken it slightly to off white. I then sprayed this hull color in light coats over the wings resulting in a nice subtle grime weathering effect that will be combined with other weathering techniques later. I exaggerated the contrast in the photos to make it show up, but it's perfectly subtle in person.
Even though I sprayed the grime texture as an undercoat to the main hull color, you could easily do this ON TOP of the base coat. As I said, the effect takes a few passes to build up, so you could do this on top of your final paint job without worrying about going to far. You could also use a lighter gray instead of the flat black I used here, and it would give a nice subtle effect. This would especially look awesome on the base coat of a Millennium Falcon.
Anyway, just wanted to share, and I highly encourage you guys to try it if you want small scale grime and speckle effects.