If you make a mistake in drawing on the paint layer, you can erase the paint. 

To erase paint from a paint layer:

1. Make sure that the paint layer is selected 
2. Press the Command key as you begin drawing over the paint you want to remove. The cursor shows a red minus sign, indicating that you will remove paint.

You can lock a paint layer to prevent it from being accidentally modified.

To lock a paint layer, click on the lock icon in the Paint Layer Effect Controller.

To unlock the layer, click on the lock icon again. The icon changes to show whether the layer is locked or unlocked.