Help with scratch off effect

Hello All,

I have an assignment for school to create an Android Lotto Scratch Off game. Using a widget I set up the menu and random money materials in a uniform grid. Shows in the image below:


Now I am trying to create the scratch off feature. Where the player will drag their finger across the screen, gradually revealing the image below. Currently I am trying to accomplish this with the HUD. Shown below:


Two for loops populate the draw material area with a bunch of small squares. My plan was to change the material properties as each one gets touched to ‘simulate’ the scratch effect.

Does anyone have a better way of doing this? This method simply is not working.

Any help you can offer will be greatly appreciated.