Using walnut shells is what VoA (Volkswagen of America, who also was responsible for Audi) used to use too. It works really well. In the 1980's a lot of German made cars had issues with carbon buildup on the valves do to leaking fuel injectors. After shutting off the engine, some of the injectors would leak fuel and drip teh fuel onto the hot intake valves. Over time this caused some carbon buildup.
I did not think anyone made a walnut shell particle blasters today. I guess it's true what hey say about everything old becoming new again. lol Acetech - I would love to see some photos of your meadia balster in action.
Dackel