OMG! I just now saw the video. I didn’t even notice that the text “What I’m trying to do” is a link. Okay, the easiest way in my opinion would be to use dynamic paint. It’s been awhile, I don’t remember if it’s called wetness or drying. Either way it’s an option in Dynamic paint. You set the B as the canvas. Add a mesh object to be the brush. For this I’d use a cube or maybe a sphere or another cylinder to give more character to the painting(AKA mask). Then parent the brush object to the cylinder so that it follows the cylinder. In outliner, click the camera icon next to the brush object so that it isn’t rendered.
For more info on dynamic paint see chapter 6 of CORE: physics course
https://cgcookie.com/lessons/adding-dynamic-paint-to-an-object-canvas-brush
