How to remove the "old pit" in Shanghai dialect is the most effective.

Use towel gourd sponge every time you take a bath (called dried towel gourd in Shanghai dialect)!

Or go to the bathroom to wipe your back!

It is best to soak in hot water for 5- 10 minutes when taking a bath, and then use loofah to stalk or wipe your back, so that the "old pit" is easier to come down.

I think the stems of loofah are too hard, so I'm using nylon bath now, which I bought in the supermarket.