I just turn the bolts backwards and forwards until they where free to turn, then I used the bit, tilted it slightly in the bolt and pulled up and twisted. Once the bolt was enough out, I could get it with my hands. Hope this makes sense.
I'm not sure what material they are, but the back of them are soft (you can see it in the images above).
I'm not sure regarding the powder coating, I know when I'll be getting my wheels refurbed, I'll be taking all these bolts out, just because I want them to stay silver.
Daz
Not sure mate, I would measure the diameter of the bolt and search for a triple square bit.
Daz