I needed a hard hat for my skeleton that will be sitting in my mine cart. My good old scrounge of a dad found one for me. It was blue and white plastic (stupid me didn't think of taking a before pic of it).
First I spray painted it metallic silver. Then I went out to the garden and sifted some dirt over a really fine mesh. I got some really fine dust. Then I took clear cover spray paint and wet down the hard hard. I layered a fine dusting of dirt over the helmet. Making sure to get it heavy in the crevices. I repeated this a few times and smoothed and bumped up certain areas. Then a fine spray of clear over the whole thing.
It looks so cool in person, you can't really see all of the bumps and ridges the dirt made in these pics. It is especially cool on the brim, as if someone's dirty hands had taken the hat off on and on thousands of times (Vladamir Guerrero would be proud).
Here are a few pics.