It is obligatory for you to remove anything that prevents water from reaching the skin prior to Wudu' (ablution). Accordingly, if you have already performed Wudu' and Salah, then found out that one of the parts of the body that must be washed in Wudu' was covered with some substance that prevents water from reaching the skin, the Salah is not (Part No. 4; Page No. 69) valid. Therefore, you have to remove anything that prevents water from reaching the skin, perform Wudu' and repeat the Salah.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.