I did a bit more looking into using steel wool to clean coatings off the lens elements.
From several patent applications, steel wool APPEARS to have a knoop hardness between 120 and 180.
MgF2 is a more common modern coating. It has a knoop hardness of 415. It is harder that steel wool. Modern coatings will wear down the steel wool.
Modern optical glass has knoop hardness values between 450 and 610. This also is harder than steel wool!
If something is hard enough to remove modern coatings, it will remove glass.
If I need to wear down steel wool, I know what to use.
It looks like I need to investigate chemical not abrasive means to remove the coating. Is there a safe solvent? Maybe I should try to find an old uncoated M42 lens.
I got the optical glass and MgF2 hardness values from Bob Atkins web site:
Photographic Lens and Filter Cleaning
p.s. I've used steel wool for 40 years to clean kitchen glassware (tumblers, Pyrex, wine glasses) without leaving a mark. I have also used it on window panes without consequences.
---------- Post added 05-18-14 at 10:34 PM ----------
Anyone know of a good 50mm-ish M42 lens for me to check out?
Thanks