Thanks for the reply,
I'm aware that it is pH dependent but to lighten the elbows and knees you need an intense moisturizer and an Exfoliant . Any suggestion on how to do that in the same recipe. I read from dermatological reviews that Urea is a perfect moisturizer and it's already being used in commercial products that claim to lighten elbows and knees. Now, if I can't combine Urea and glycolic acid in the same formula, what substitutes you suggest to achieve these two goals "exfoliating and moisturizing effect enough to treat dark elbows".
Your help is very much appreciated.