being in the hvacr biz, we wash coils on a routine basis, and the product that i like the best is nu-brite. it's a dilute sodium hydroxide(lye), that you mix with water. usually at a 25% solution. there's a bottle and spray handle that'r part of the package, and it throttles down the hose pressure but floods the coils with the solution. it's a self-foaming product and depending on how dirty the coils are and your mix strength, you can watch it bubble the dirt out from in-between the fins. but caution is needed - get it on your skin or in your eyes and you'll know it. gramma's old lye soap had a way of taking off skin to make sure it got the dirt to come with it. i'm careful, so i don't bother with gloves, and my hands are as clean as pink at the end of a day. i do wear goggles, bec a little wind and mist will reduce you to painful tears. and just a few days ago, i ran into a guy that was in the biz at one point, and he and his father discovered that it's excellent for cleaning greasy blocks etc. but in either use, it will permanantely change the appearance of aluminum, and with coils, it must be rinsed very well with plain water, otherwise it'll leave a residue
and there are also kinder, gentler, greener products out there, but......
http://www.nucalgon.com/products/products.htm
