To get in those hard to reach places in the engine bay, those sponge-on-a stick things for the shower are great. Since you can air dry so many things, just be as sloppy as need be. And yes, there is lots of scum that you'll just never get rid of. My valve cover has tons of stains, and many lower bay parts are covered with dust and grime that I'll never be able to remove by hand.

 

This was intended as a reply to the post: Hose that goes to nowhere. Oops, still haven't gotten the hang of this interface.