How to make scratch effect (like in lottery tickets) with Unity3d?
(Sample games: Scratch Kings, Scratchers)
You could place an image to be scratched away on an object infront of the thing to be revealed, have a material with a special texture on it and the use SetPixel to set the alpha level to 0 on some pixels where you want the scratch to appear. You would need to do some maths to get the right position in the texture to update.
If anyone still interested, I advise to look http://www.arongranberg.com/unity/unitypaint/ , very useful