I'm about to paint my grenade and want to have the bumps green and the part in-between black. Would I do the black by spray painting it then wiping it off?
Look at Adam's armor, he achieved the same exact effect you'd like by spraying his base color of green, then quickly spraying black and wiping it off with an old sock.
or you can undercoat black and let it dry.
Then put toothpaste in the cracks, then spray the green, then wipe off the toothpaste.
Gives it a nice, cracked, 'used' feel.
Mmm, toothpaste..