Taylor,
I do not think it was tow causing trouble in the bore, it is Very much more likely to be the interim years, between use and arriving in our hands.
The guns Keith Neal wrote of in the Packington story, were completely rust free both inside and out in most cases.
These guns are of the vintage of which we speak here.
The no -rust outside was due to being kept "anointed' with deer tallow; An excellent preservative.
So yes, I am sure the rust we see is the years of neglect and even living in the barn or loft/attic, than caused by faulty cleaning practices.
I have no antique rifles that show no rust in the bore, until we get to the 1850's that is, not earlier, but smoothbores yes, mirror bright and not cleaned up later, and these would be cleaned in the same manner, with tow. Tow I still use .
Robby,
If you start one of your sheets with a sharp knife, it should tear down full length at that width of your patch, then only the cross cuts to bother you.