Tricks to repair window sill?
That's probably just several layers of paint or paint plus a specialty undercoating for trim.

Chip it off, ensure there's no mold, remove any rot using a wood chisel, patch with a good quality wood filler, sand to 100grit or better, seal with a good primer/sealer, and repaint. If you find mold it's probably easier to just replace the sill, bleach and most mold killing products will not work on wood (including mold killing primer)