ActionScript 3.0 :: Unmask Image Using MovieClip With Paint Brush Effect

I am trying to unmask an image using a movieclip when the mouse hovers the stage. (like a paint brush effect).
my_mc = image
my_mask = mask movieclip (square)

this is what I have so far

ActionScript Code:
package {
import flash.display.Sprite;
import flash.events.MouseEvent;
//Main Class
public class Artist extends Sprite{
private var drawing:Boolean;
[Code] .....

I tried putting the masking property in the constructor ( image.mask = imagemask ). but that gave me an error because imagemask is defined in the draw function. If I define imagemask (privately) ontop of the constructor and add the masking properties to the constructor it doesn't work correctly.e