Yes I have used it on my exhaust system by soaking the cast iron parts for a couple of days in ca 10% EDTA solution during which I also assisted by chiseling and application of a MAP gas blow touch.
The crud in these fittings is a composite of carbon, hydrocarbon residues from exhaust gasses (somewhat combustible), calcium carbonate and iron oxides.
My cast iron fittings came back to grey bare metal although heavily pitted from corrosion. The motor is 10 year old with 500 hours on the clock.
I have also dip coated the cleaned fittings in an epoxy paint - Amerlock 2k and when properly cured will resurface the mating faces by rubbing those faces with 800 wet/dry paper held against a price of flat glass.
Amerlok 2k is widely used as a protective coating for structural steel and required minimal preparation. It sticks well. forms a very hard surface and mix ratios are not so critical.
Hope this helps.
PS I have also coated the stainless ends of the waterlock silencer.
