When you're editing your content, you can simply 'swipe' over to 'Mask' along the bottom toolbar. You'll see four circles. The first will 'erase' completely, the second will erase partially, the third will 'add back' partially, the fourth will add back completely. 

Alternating between the first and second circles you can start erasing certain elements on a specific layer on your project! 

Note: You can also pinch-to-zoom to get all the tiny details! 

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