I used Evaporust in my cooling system a few months ago. I filled the system and ran it till it got warm. I left it in the system for about 1 week and ran it a couple of times to warm it up, but not over do it. When I drained it, it was black, which according to Evaporust means it is spent. I bought mine at Tractor Supply when it was on sale, I had to visit multiple stores as they only had one gallon in each.
I also was replacing the water pump at the time. I then rinses several times with distilled water before I put in the new wate pump. After I did this, the system did run cooler. Before it was approaching 190 and a bit more when going up hills and that was in 65 degree weather. Now its much hotter out and stays below 190. So I don't know if it was improved because of the Evaporust or the new pump or both. I think the new pump is more aggresive than the old one which had started to leak. Also when drained, there were no pieces of rust or flakes, just dirty evaporust.