set tooltip on a wpf grid?

gistfile1.txt

thought this would work

            ToolTip objTooltip = new ToolTip();
            //set background color 

            objTooltip.Background = new LinearGradientBrush(Color.FromRgb(100, 120, 130),

            Color.FromRgb(200, 220, 230), 0);
            //Set border color 

            objTooltip.BorderBrush = Brushes.Red;

            objTooltip.BorderThickness = new Thickness(1);
            objTooltip.Content = riskTag.Signature;
            objTooltip.HasDropShadow = true;

            objTooltip.Placement = System.Windows.Controls.Primitives.PlacementMode.Bottom;
            g.ToolTip = objTooltip;

            ToolTipService.SetShowOnDisabled(g, true);