> I know, but what I want to do is to modify the alpha values of the
> affected layer.
> When I apply Mask to a Layer the alpha values are saved
> permanently so you can't paint anymore.
Painting with colors works and and painting with black to have
transparent doesn't work anymore if you apply the mask.
So create a new layer for to paint with colors and don't apply the mask
before you finished.
>This is really the problem and
> the only solution I found is to use Levels or Curves to modify them.
levels == layers ? Then you're right.