I've been working on an aluminum W.I. for several weeks now and was needing something to clean in the valleys and peaks to try and remove 80+ years of baked on crud. I had to make a trip to the local hardware and came across this abrasive impregnated nylon bristle brush. I bought the blue one (240 grit) simply because I wasn't sure what effect it would have on the aluminum. To my surprise it worked great. I'm not sure how it will do on cast iron but I have some experimental pieces to try on. I've attached the link for anyone interested in checking it out.
http://dicoproducts.com/nyalox.html
http://dicoproducts.com/nyalox.html