Protecting the palm-rest area of a MBP with a non-permanent cover
I love a clean mac and have seen all sorts of ones with all manner of paint, grease, and plain old skin oil and dirt. I would worry about the keyboard more than the palm rests. (the area with speaker perforations and the keyboard tray area)



I always start with plain water to clean, then move to either alcohol or a light detergent (very watery mix) with no colors or scents if possible. I use that all over the mac - but the aluminum area you indicated is both solid and not affected by much stronger cleaners. I have seen macs with nail polish and acrylic paints come clean in that area.

As long as your cloth is damp and not dripping, even cleaning the trackpad area won't introduce liquid to the battery bay. It's of course best to have the mac off, just in case there is a spill to avoid a short.