Apart from the fact that you can spin-kick the fireballs back at the giant robot, and simply avoid the thinner bomb explosions, what exactly do you need to aim at with your paint?
(I'm assuming there is a paint solution, because there's ALWAYS a paint solution, even though Gus keeps yelling about how you need to thin his armor. That can't be the only way, though I've noticed you can thin the outer face a bit, even though nothing ever comes of it.)
So, since I've demonstrated that you can run around for 15 minutes lobbing back fireballs, and make zero progress, I'd like to know what major part of this I'm missing. In a thread below, someone mentions filling up tanks with paint or thinner. Are they talking about the things that look like ears on the giant head, or what?