2012-07-08 // 20:22:34 jcoldslabs As far as I understand the white band (or light-colored band on the color films) occurs because of the pressure each alternating chemical pod "bulge" places on the sheet below it. If you take a look at how the film is packaged, every other sheet is facing the opposite way from the sheet above. The sheets are all flat EXCEPT for the slight bulge of the chemical pods. I bet the white band corresponds to the spot where the pod from the sheet above was pressing down. In fact, I bet ALL the bands we get with these films are in the exact same spot and are the same width. This would indicate it is not user error but a storage problem with expired film. When film was used before it's expiration date (ha!) the bands did not have time to develop (no pun intended!) This issue has only come to light with the use of very expired films.
Anyway, I found found the above to be true in my case for a variety of my 4x5 instant films. Pulling speed through the rollers matters, but does not affect the band. Overexposure, as you've seen, does help a bit.
2012-07-08 // 12:20:07 g. I've been wondering about this white band myself a lot, and wrote about my insights here to calicant: »link
In addition to what's written there, I believe it's that the consistence of the reagent in the pod changes with age, and thus, makes the film more prone for this problem. Since, I began to pull faster and try all the 545 backs that accumulated over the years, and can confirm that with some, I always have it, with some sometimes (depending on the pulling speed), and with some nearly never.
Eventually, it could help to warm up the pod to make the reagent less squishy (if it's that way round), but I haven't tried out that yet.
Best, g.