Controlling Mask/Alpha Amount

Hello,

I need to display parts of a lower layer using either alpha or mask, but I can't find a way to limit the amount of the lower layer that displays thru. I only need for the areas in the lower layer to partially show thru the upper layer. I can see how to fade the upper layer so that the bottom layer shows thru, but I only need portions of the bottom lower to partially show thru. Let me explain how I would do the iin Photoshop:

1. I would setup my top/bottom layers.
2. I would add a mask to the top layer.
3. I would load the alpha channel of the image that resides on the bottom layer.
4. I would select a brush and set the color to a light gray, then paint on the top layer's mask to partially expose the portions of the bottom layer.

How can I do thin in AE?

thx