I either forgot how to do this or can't find a tutorial just yet, and I'm used to doing this quickly in Flash.
I want to make a small, rectangular mask shape the uses position key frames to slide across and reveal a layer of static text and solid rectangular color fill. The text and solid fill background were imported from a PSD file and are on two different layers.
I started with a rectangular solid black layer, added a mask to this shape, but I'm not able to set this mask to invisible and mask off the two layers (text and color fill) below it.
Of course, I don't want to mask off every thing else, like the color background (imported from PS.)
I could go back and re-do teh PSD and just animate the text and color fill layers, but I was trying to do this with a mask.
Effect --> Transition --> Linear Wipe? Blending mode: Stencil Alpha and animating the solid? Individual solids applied using the matte modes? Sounds like you're too bent on doing things the hard way because you are locked into the Flash mindset.