Why did I get a poison oak & ivy rash after using Tecnu?

If the poisonous oil has already penetrated the epidermal layer of the skin and bonded to the deeper skin cells, it's too late to stop the reaction from occurring. The benefit of using Tecnu Original Outdoor Skin Cleanser is if there is still urushiol oil on the top layer of skin, Tecnu will remove it and prevent further contamination.