How can I “paint-with-opacity” onto a layer in gimp [closed]

Question

I have a layer that is at 50% opacity. But in certain regions, I want to use a paintbrush to bring the opacity higher. How can I do this?

Answer

1. Set opacity to 100%.
2. Add layer mask.
3. Fill the mask with 50% gray.
4. Draw into mask using black and white any way you want.

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