Mold/ wet basement window sill

[Larisa Van Valkenburg]

Hi,

we fixed the basement window sill a year ago, it was ruined by water leaking from outside as we thought.   I had handyman work on the slab to raise it. The basement window is on the north and irs quite damp outside in the corner there.

Today I saw black mold on that nice window sill that has not been painted yet. I had a contractor to look at it today and he told me to rip it off and to insert some plastic to prevent it from getting wet. 

I do not want to redo the window sill for hundreds of times.  I would appreciate all your thoughts and guesses. And the best way to fix it once and for all,thanks