I'm doing some masking with the brush tool. Each keyframe I'm drawing a bit more of the mask. My problem is if I go back to the first keyframe and paint something it doesn't appear on the other keyframes.
I thought I found the solution with Edit Multiple Frames, but it doesn't seem to work with the brush tool.
Editing multiple keyframes the way you want is not possible (all the other tools like drawing squares etc. will only appear on the last frame, even if mutiple edit is selected).
Want You want to do is going backward: say: your mask is ten frames long, now navigate to the tenth frame insert a keyframe (you should now have 1 keyframe on the first and one on the tenth, both empty); now draw the last stage of your mask on the tenth frame. Then hold down the Alt key and drag the 10th to the 9th frame. Erase the parts you don`t want hold down Alt-Key and drag the 9th to the eight frame and so on.