Help with animating a painting effect

I am trying to do what I think is a simple animation but I can’t get it to look right. I want to the animation to look like a mural is being painted on a white canvas paper. I want this to happen automatically and do not want to use any action script.

So far I tried the following:

  1. In the bottom layer I placed the white canvas layer.
  2. Above that layer I place the mural.
  3. I masked the mural.
  4. Then I tried a several shape tweens moving a a circle shape diagonally stretching from the top to bottom to reveal the image underneath.

Now I have several issues with this. First of all this does not look like it’s being painted on. Second, if the shape tweens touch, the start to morph.

I found the following painting reveal technique and would be happy with something like this http://www.tutorial5.com/content/view/115/46/ but this tutorial involves a mouse controlling the painting effect. I want it to be automatic.

Also I saw this component, http://activeden.net/item/autodraw-dynamic-hand-drawn-graphic-pencil-effect/47185
A similar revealing effect like the airbrush would be good.

Please advise