Are you old enough to remember when Prestone sold a two-part can of cooling system cleaner? One section had the very aggressive cleaner, the other section had the neutralizer. That stuff would scour your cooling system like it was steam cleaned. I haven't seen it in years. What I use now is Cascade dish washer soap. It will make the cast iron look like new, but will attack aluminum, so you can't leave it in too long. Baking soda will neutralize it, then lots of fresh water. Both the Prestone and Cascade will find and expose any leaks so be prepared for that.