The two bathroom basins in my 1986 house
(finished by a handyman-occupier) are different.
Downstairs there is a rubber (?) gasket between
the basin and the tile surface into which it sits.
Upstairs there is no gasket (or caulking) -- so
moisture creeps round the edge of the basin
at one point, then drips down underneath.

What can I do to fix this leak?

--
Don Phillipson
Carlsbad Springs
(Ottawa, Canada)