Hey,
sure. It´s very simple: I used spray paint from Edding. Edding Permanent Spray 5200.
You can use other brands, I just like the fact that these are matte/semi-matte.
They are quite pricey, tho.
Grey color is Antracite (I believe) and then I used yellow for bits and bobs.
I didn´t prime the case at all; I just washed it and made sure there was no grease on it.
Then simply paint, apply scratches with steel brush or whateven gives the effect you´re after.
The paint sticks pretty well to the case, and since the case is stationary, and you´re going for a weathered look, there´s really no need to prime.
No need to credit me :) Just glad I could be of inspiration. And FWIW the case still looks the same 2 years later, so it worked out fine.
Happy painting!
Lasse :)