You know in Photoshop, you can easily blur the edges in a layer mask using, amongst others, the Gaussian blur filter. But what if you want to harden an edge or a part of an edge?
Well, have you ever seen the brush blending mode “Hard Mix”? Located in the submenu right to the next of the “Opacity”. If you select the “Hard Mix” blending mode, you will see that your brush behaves quite differently than normal.
In fact, when you try to paint in black, you can only paint in areas where the colour underneath (usually white or a shade of gray) is not pure 255-white. That means that you can remove gradient in the mask by paint black on top of them (because their RGB value (which is the same for all three channels) is lower than 255). This principle applies also when painting with white or any other colours. I usually use this to refine a layer mask. You can see below a GIF of this working live.
I hope this trick is something you have found interesting. Do you have some other layer masks tricks and tips? Tell us below!