Masking paint brush solid layer

Howdy,

I searched the web to figure out my issue but couldn't find the right answer anywhere.

I feel like it should be a very simple solution.

I am drawing animations with the brush tool onto a black solid layer which is set to add blending mode.

My goal is to create a mask on the solid layer to hide a certain area of what I drew... but it doesn't change anything.

I noticed when I add another effect to the layer (such as curves) the mask does hide that area of the effect, but thats not exactly what I'm trying to do.

Thanks in advance for anyone who can give me a tip

ps. I have After Effects CS6

-Alex