Spray the area first with your general purpose cleaner. For bath rings and scum, allow the solution to “dwell” for a few minutes. Spray again if necessary.
Using your baking soda paste, smear the paste across the bristles of a soft cleaning brush and completely cover the area with the paste. Use paste liberally.
Run some water in the tub as you begin washing the area with the brush. Finish by wiping the area clean with a soft wet cloth. Repeat if necessary.