i have a friend that has a big concrete fountain that she wants to mosaic. but, she recently discovered a leak in the basin and she asked me how to repair it. i havent worked with concrete but told her i knew where to go to get answers, so i am asking you guys. her brother in law told her to coat the inside with mastic but i didnt think that was right, so i told her to use either quickcrete or thinset, but i am not so sure i am right either. does anyone here have a fix thats not too complicated.