I typically use MEK to clean the surfaces before spraying primer. The problem is that a lot of very fine aluminum dust gets embedded in the skin. I found that I can wash the parts a few times with an anti-grease dishwashing soap and water. After this, I wipe down the skins with MEK, then I spray the primer. I'm not sure if this is a recommended practice or not, but it seems to work well for me.
Steven roughing the insides of the VS skin with a Scotch Brite pad
Washing the skins with water and Dawn dishwashing detergent
