Because I do customer's loops, they need to be free of crap or they foul my CBN wheels.
A lot of guys get their undies in a bunch, when mentioning ’chain cleaning’. And, in many cases, it is unnecessary, for a light, basic sharpening.
But I have worked with a lot of used, abused, and damaged chains. And the first step in those cases is to clean off the gunk to see what I am working with.
Sometimes it reveals damage, or improvised repairs. Sometimes rust. Brand markings. Lots of interesting things.
It does make for a nicer looking chain, and helps oil get to the center of the rivets, which act as bearings.
I have posted most of that in my ‘Chain Salvage Challenge’ thread:
https://www.arboristsite.com/threads/philberts-chain-salvage-challenge.245369/
Tried a number of things. Lots of ways can work, depending on preferences and what is available.
My current preferences are Super Clean (or equivalent), to clean, along with old toothbrushes and an old, plastic, kitchen cutting board (protects the chain and concrete wash tub).
Citric acid (food grade powder, on line, 1-2 Tablespoons per gallon of water) for surface rust. I use a fine, stainless steel ‘scratch brush’ (looks like a toothbrush) from a local welding supply shop. About $1/ each. Work better than the $3-$4, brass, ‘refinishing brushes’ from the home centers.
Philbert