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I would tend to thinks there's less graffitti on tankers, probably because the places where these cars are loaded/unloaded are refineries, chemical plants, etc. where security is much tighter.
Of the ~30 random sample photos I viewed of GATX tank cars at rrpicturearcives.net, I only saw 1 that had even a hint of graffitti on it (I'm definietly NOT complaining, LOL)
In addition to the above, a tank car just doesn't provide the nice flat suface of a boxcar or covered hopper. It's harder to make graffiti look good on a cylinder. I've seen some that were pretty well covered in tags but a lot less than the average boxcar or covered hopper.
I have also noticed this as well and attributed it to where these cars are kept. Higher security areas. I presume this is the same reason you rarely ever see it on coal hoppers as well.
There's a Propane area in my town that has 3 cars sitting there almost all the time. The cars are in the open & they are almost white. The propane tanks & the pumping station are fenced in. The last time I went by there the ends of the cars had a lot of Graffitti & some in the middle.
I've only seen one that was completely covered w/1 scene & that car was almost new.
A white background is a nice canvas. LOL
I see a lot of tank cars out on the open and it is rare to see a lot of graffiti on them but as stated earlier, they do not sit very long so there wouldn't be a lot of time to get the work done. There may be the occasional small tag but, nothing too extensive.
Compared to other cars, I've seen very little on tankers - usually just some random scribbling. Dunno, aside from security, tankcars around here tend to be parked in nasty, stinky places that I'd hesitate to go into. The odors in the air just sort of say, "Stay away." The exceptions are the cornsweeteners.
I've seen them sitting around out in the open although I think it'd be harder to tag them since they're cylinder shaped and don't have a flat enough surface like a boxcar or even a covered hopper. Usually if they get hit it's simple stuff like random scribbling.