20D IR filter pic above was the combined result of:
b) advice that cleaning is foolproof
c) grit on the sensor
d) incorrect tools being used
e) testing how easily the filter is to scratch after the initial scratching occurred
I'd say the surface of the filter was stuffed on the first wipe given the way it happened at the time, but common advice at the time (2005 I think?) was that "it's just a piece of glass, if the dust doesn't come off in the first wipe, just keep at it"
of course once the damage was done, I did quite a bit of testing on the scratched filter & found you could scratch/remove the top surface fairly easily with just a fingernail. Suspect it depends on the camera model as to the durability of the filter - certainly there is a fair bit of comment on the 5D filter for example, being easily damaged.
Tom, I believe your comment "It's one of those things that is so simple and easy that you can do a great job even if you have no prior experience or practice" is overstating the case somewhat...
some fairly old canon camera stuff, canon lenses, Manfrotto "thingy", 1D MK II converted for IR, and now an M5
TF posting: here