I had this happen before when I was first learning ArchiCAD. What happened is that I could see onscreen the sketch rendering for a split second before it would be covered with white, and then the rendering was over. I can't remember what, exactly, I did to fix this as it doesn't occur anymore. But, one quick fix for me was to hit the "stop" button about halfway through the rendering process. It would then give a full sketch rendering without applying the white over it. I know it's not the best solution for you, but may work until somebody else can provide a real solution.
If you already do renderings with Lightworks or the Internal Engine, you might just prefer to render that way and then use AKVIS sketch instead. It works as a Photoshop plugin or stand-alone. There is a free trial, check it out.
http://akvis.com/en/sketch/index.php
Foster
