One of the conditions of tahaarah (purification) being valid is that the water must touch the skin. If there is any barrier such as grease, paint, wax or glue that prevents water reaching the skin, then the tahaarah is not valid and prayers offered in that case are not valid.
The evidence for that is the words of the Prophet (peace and blessings of Allaah be upon him) to Abu Dharr (may Allaah be pleased with him): “If you find water, then let it touch your skin, for that is good.” [Narrated by Abu Dawood (332); classed as saheeh by al-Albaani in Saheeh Abi Dawood.]
Imam al-Shaafa’i (may Allaah have mercy on him) said:
'If there is on him any mastic or anything thick that will prevent the water from reaching the skin, his washing of that part for wudoo’ is not valid, unless he removes it or removes enough so that he knows that there is no barrier to the water touching the skin.' [Al-Umm (1/44).]
Al-Nawawi (may Allaah have mercy on him) said:
'If there is wax, dough, henna and the like on one of his limbs, which prevents water from reaching any part of it, then his tahaarah is not valid, whether the amount is large or small. If there are traces of henna and its colour left on the hand, without there being any solid material left, or elsewhere, or traces of liquid grease whereby water flows over the limb but does not stay there, his tahaarah is valid.' [Al-Majmoo’ (1/529).]
It says in Fataawa al-Lajnah al-Daa’imah (5/218):
'If the colour is impermeable, then wudoo’ is not valid unless it is removed before doing wudoo’. If it is not impermeable, then wudoo’ is valid, as is the case with henna.'
And Allaah knows best.