So far I have sprayed and used a tooth brush (is this too harsh) for the Z-9, and stopped b/c the car was getting too hot, so I pulled it in to a cool garage and decided to see if there was a better way.

 

I would not use a tooth brush or anything with still plastic bristles, I would use a horse hair brush and a clean white terry cloth to wipe off the Z-9, the reason to use white is that the cloth will turn brown or black and tell you how much dirt you have removed, and then simple wipe on the Z-10 and work into the leather until you can not see the Z-10 any longer and thats it.