WPF - Make scrollbar fade in / out

WPF - Make scrollbar fade in / out

I set my scroll bar to be hidden when you are not over the ScrollViewer
and only shows when you hover over
This is the code I am using - it works great
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="False">
<Setter Property="ScrollViewer.VerticalScrollBarVisibility"
Value="Hidden"/>
</Trigger>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="ScrollViewer.VerticalScrollBarVisibility"
Value="Visible"/>
</Trigger>
</ControlTemplate.Triggers>
My question is, how can I make the scroll bar to fade in and out instead
of just show/hide right away?