Boxcars used for bagged flour loading were most often "lined" with with heavy craft paper, this was for two principal reasons, 1. sanitary - it kept the flour bags cleaner, 2, to reduce damage to the flour sacks, the heavy paper covered over minor rough areas of the interior car lining that had a tendancy to catch on and tear the flour sacks. Most often the lining consisted of rolls of heavy paper which were unrolled and tacked up on the sides and floor of the boxcar. I never saw canvas used as car lining as it would be expensive, but I certainly cannot say in never happened. When flour cars had to be fumigated, canvas would often be used across the doorway area to help the fumigant not all escape to the atmosphere.
The Kennedy Car Liner company made a pre-folded paper liner which could be unfolded and efficiently and quickly lined half of a 40' boxcar, so two car liners did the whole car. The Kennedy liners were also used on boxcars for bulk flax loading, the nature of flax being that it would flow like water and leak badly from even the smallest hole in a car. In the 70's Western RR Assn. graindoor dept. still had a large inventory of Kennedy car liners, which by that time were not seeing mush use. I used a bunch of them to line boxcars in wood chip service which had been pressed into use to move a huge order of malting barley from Twin Cities to Mexico.
The RRs supplied the car liners or material, but the flour mill or elevator had to pay RR is they wanted them to install them.
