procedure InvalidateDesktop;
begin
  RedrawWindow(GetDesktopWindow, 0, 0, RDW_INVALIDATE);

  //if that doesn't work then try this:
  //Sendmessage(GetDesktopWindow, wm_setredraw, 1, 0);
end;

FBitmap.Canvas.Brush.Color := clWhite;
    FBitmap.Canvas.FillRect(FBitmap.Canvas.ClipRect);