Dats da problem with poor workmanship when installing the pipes. To be honest, in my opinion, any solvent, silicon or wateva so called waterproofing is just temporary. Best is still to rectify the piping concealed in the wall.

Tiling den god bless lo. Waterproofing is best done when doing concreting of slab. Anything else after that would not be as effective. Final solution, repaint the portion and sell off the house.