Hi
About the 'powder', it depends whether you want good quality ones or lower quality ones...
For the lower quality ones (much cheaper), you can go to bookstores and get chalk pastels (not oil based ones). Scrab the pastels or sand them to get fine dust and use the dust to weather the vehicle.
Higher quality ones are those pigments powders manufactured by companies, there are 2 good ones: "MIG" and "MMP". The pigments are much more finer and their range of colours are wide. Of cause, they are more expensive.
I cant help you on the texture thing...I read somewhere about 'painting' liquid cement onto the surface and then while still wet, use a stiff brush to dabble onto it.