I don't know of any special 'gearbox paint', but no paint will stay on for long unless you can thoroughly de-grease the outside of the box, and remove all traces of the white powdery corrosion. Wire brush is not enough, I tried it that way, it looked spotless, but the paint (VHT Engine laquer), flaked off in those areas within a couple of months. I think shot blasting would probably be the only way to remove all of the corrosion.