This is propably a really simple thing to you but i've been googling solution for 30 minutes.
So, when i mask my hand to be on other video (rotoscoping?) i mask all the frames. But my problem is that when i delete mask from some keyframe, it affects every mask in the layer. So when i delete mask from one keyframe, it deletes it from every keyframe.
You'll need to animate the Mask Path property. Examine what you're masking very closely so you can cut your mask(s) appropriately. Learn how multiple masks interact with one another: AE Help can be... well, helpful in that regard.
And good luck -- this is mind-numbing, painstaking, time-consuming work.
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA