In the meantime, i discovered that a redraw is triggered when i the renderer
object changes.
For now i use this workaround:
Dim temp As RendererBase = Me.SandDockManager1.Renderer
Me.SandDockManager1.Renderer = New myRenderer()
Me.SandDockManager1.Renderer = temp
I'm sure the is a better way to handle this issue, but for now... 
I also have a new issue:
Is it possible to determine the currently drawn/measured TabbedDocument object?