when I have flushed with the garden hose to begin with, then added the Iron out, ran engine to temp, circulate it good, then flush again whine running til clear, I was blown away by what that stuff would work loose.... some times if I had one that was really bad I'd let a car sit overnite with straight water plus the Iron Out so it could soak in... run engine when 1st added to distribute/dilute it into the water, let sit, then warm up engine again and THEN flush once again til clear.... in 2 stages of flushing with the hose I might run 30-40 gallons of straight water thru an engine but so what? It works for me.
(hey Iron Out was made to clean boilers and radiators like youd see in an old school house and at one time Prestone used to sell a 2 part powdered radiator flush and the ingredient in Part 1 was exactly the same as in Iron Out-- Oxyllic acid-- so why not use this stuff?)
I can understand that as a shop owner you cant spend that much time on any given car and make anything.....
Ive also seen radiators so bad that no amount of on car flush would clean them.... blocks though, this stuff works pretty well.
I had 1 318 in a van once that I got a call about an antifreeze leak, wound up being a freeze plug.... once I popped that old leaking plug I saw that the leak was actually slowed by the deposit buildup in teh water jackets, the leaking liquid would have to filter thru all of this crap to eventually find its way out.
I popped all that I could get to (2 on front of block and all of them down the side) and pulled the block drains, poked my way thru those, set the hose in the water neck and gave it He11..... I flushed that one most of a day with freeze plugs out.... nastiest Mopar I have ever seen. I then wound up replacing the water pump, T stat, radiator, every coolant hose and those freeze plugs.... at least in the 70s those Dodge vans had tons of room from underneath to access teh freeze plugs.
It ran plenty cool from then on!