You can use Lemon and Potash to lighten your dark Elbows, Knees and Knuckles because, first of all, what matters is what the ingredients contains

Lemon(as a Skin brighter) you can check out how to get rid of Dark Elbows, Knees and Knuckles Fast naturally at home
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It is rich in vitamin C and citric acid. vitamin C is a great antioxidant for neutralizing free radicals and boosting collagen production. This means that Vitamin C can help lighten age spots, dark spots or a fake tan gone wrong.

Lemon (as a shine eliminator)

Lemon is great for reducing excess oil

Lemon (as an Exfoliates)
It can be used as lip exfoliator

Tip: Put a little Lemon juice on your lip before bedtime and Wash it off in the morning to help remove dead skin cells and dried skin.

Warning: Do not try this if you already have severely chapped lips

Lemon juice acts as a natural exfoliates as it contains citric acid which helps to peel and remove the top layers of the dead skin cells


Potash
https://www.youtube.com/watch?v=2_c7ddrSoVg

Potash is edible and is used for cooking.
it's used for treating toothache
it's used in administrating cough treatment
It's used for fertilizer production
Potash acts as a preservative

You can check out how to get rid of Dark Elbows, Knees and Knuckles Fast via:.
https://www.youtube.com/watch?v=2_c7ddrSoVg