this techinique is courtesy of fiasco.
painted the base coat and carried out normal weathering on that (dry-brushing + washes), then I made a mix of white pastel chalk dust and water. Slop it on, literally! You want it to look field applied so just get it on there!
Then take some blu-tak or putty-eraser and carefully remove the chalk from the areas you want to show wear on. A sort of reverse dry-brushing technique.
The beauty is if you take off too much, just slop more on.
If it goes horribly wrong, just rinse the chalk away with water.
When you've got it how you want it, seal with clear varnish
Here was his result
