SL 4 Scrollviewer scrollbars -- how to hide

Question

Hello,
I am trying to hide the scrollviewer scrollbars surrounding a Canvaa before creating a WriteableBitmap:

if (thing._scrollViewer.ComputedHorizontalScrollBarVisibility == Visibility.Visible)
{
    thing._scrollViewer.HorizontalScrollBarVisibility = System.Windows.Controls.ScrollBarVisibility.Hidden;
}  
WriteableBitmap bitmap = new WriteableBitmap(thing._scrollviwer, new ScaleTransform());  

However, the scrollbar is still visible in the bitmap.
I suspect there is another step to take before creating the bitmap...
Any hints are greatly appreciated. Thanks.