ANSWER:

If it is making a layer and a barrier over the skin that prevents water from reaching, you must remove it. If it can’t be removed, you must, after making ghusl or wudhu, perform tayammum for the areas missed. 

QUESTION: I applied chemical henna without knowing it may form a barrier. I did ghusl the next day and prayed, but on the 4th day I learned it might block water. I tried removing it, but some remains and is very painful to fully remove. Do I need to repeat that ghusl? Are my prayers after it valid? What should I do if it can’t be fully removed? And Should I inform others who used it or stay quiet?