#21. "RE: Offical 2011 Eastern Sierra/Owens Valley FAQ/Q&A" In response to In response to 20
I do agree with Michael, glass is 100% the way to go. I've done 60 second exposures (ND not Gradual) with my Sunpak ND8 (glass, .9 stop) with absolutely no color casting at all. With my B+W (not multicoated) resin ND4 (.6 stop) one a 15 second exposure gave me a slight halo (fixable in photoshop).
Singh-Ray switched to resin a while back, they cited "people kept breaking them" as the reason for the switch. Having said that I have not heard of any major color issues with them, however nobody that I talked to had done any long duration shots with them.