Creating Roughness

Steps:

1. Create an empty layer and place it underneath the barrel layer.
2. Fill the layer with a light-grey color using Shift + Backspace.
3. Add a Filter Layer > Adjust > Desaturate above the barrel layer.
4. Add another Filter Layer > Adjust > Curve Adjustment.
5. With the "Curve Adjustment" selected, press F3 to open the filter layer and adjust the curve. The darker areas of the roughness map represent a shinier result, while white is more glossy.
6. If you're happy, Export the image at the same location as the barrel_albedo file. Disable transparency since we don't have any.