We for months have been having issues with color streaking due to buildup in the 2nd BTR unit. We had a long stading (several months) service call with Xerox to try and resolve it, that reached the hands of technicians, specialists and management with no solution provided. In fact, despite the fact that these forums are loaded with people having the same problem we were, Xerox continually wanted to blame the paper we were using, when in fac,t we were having problems accross both digital, non digital, coated and uncoated stock. Poor customer service is putting it lightly.
What i did notice is it happened primarily on longer runs and runs with minimal coverage. Those seemed to be the most common factors. What it seeme dlike to me was not that the 2nd BTR unit was failing so much as it being loaded with excess tonor was a SYMPTOM of poor filtration and removal of excess toner. It is no secret that these machines run very dirty.
We tried some work arounds on our own and we found a solution that works every time now. We put a strip of 4 color black on the lead edge and side of our layout and we have no problems now. Same paper, same run sizes. This gives the machine an opportunity to use some of the excess toner it drops that would otherwise be floating around inside the machine. Below there are photos of the problem and the fix. We hope this helps out some other folks that might be having similar issues, with no resolve from Xerox.
Hmmm... not really a solution - more of a workaround, I'd say :-) However, I do generally agree with your findings which are broadly similar to what I'm finding on our V180 (and the V80 before that). Another workaround is to run the machine in "skip pitch" mode, which will avoid the issue, albeit running at under half the rated speed hence I do not call that a solution eithere.
I've not had the chance to check, but I presume these colour strips cannot be added via Impose? We use a 1up PDF -> Impose workflow. Also, obviously this doesn't help with non-bleed printing on A3/A4 for example.
I know Xerox blame the paper and it sounds like a bit of a cop out, but paper stock is a factor. Granted, it can eventually happen on all stocks, but in my experience it triggers on coated stock more quickly to begin with. And, for example, using two different 300gsm silk stocks, we find that one of them will trigger the 2nd BTR issue within 200 sheets (even directly after a new 2nd BTR was fitted!) and the marks are really much worse spanning entire width. Printing the same job on a different brand of paper, the problem takes much longer to appear and when it does it is not as bad. That said, the worst stock was a Xerox certified stock, so they cannot really blame the paper if it happens on their own! I am sure there is no magic paper stock that avoids this issue completely.