Just goofing around trying some new brushes and new techniques. When I first started painting in Photoshop, I wanted to replicate the feel of traditional media as closely as possible, partly because that's the look I wanted for the Captain America covers I've been painting, but also because some digital paintings look sterile and artificial to me. However, there are things that Photoshop can do that can't be done any other way. I've been consciously avoiding these digital techniques on most of the published work, but I'm doing more and more of them in the speedpaints.
I'm not sure where (if anywhere) this may lead, but I'm having fun experimenting.
By the way, I didn't reference any tank when painting this. It's completely made up, so try not to laugh too hard at how impractical or stupid it looks.:)