Control.Invalidate;
---
Also, the canvas has ClipRect;;
Canvvas.ClipRect := TREct(?,?,?,?); Canvas.Clipping := true;
----
You can assign a clear brush to the background of the parent if you are going to repaint the background yourself.
Also the use of ExcludeClipRect can disallow a rect to not be painted.
There are many ways..