That should be an indication that the Folex pigment ink compatibility is not optimal. That said, a good saturation on backlit film usually asks for longer drying times.
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Met vriendelijke groet, Ernst
340+ paper white spectral plots:
http://www.pigment-print.com/spectralplots/spectrumviz_1.htmupdate july 2012: Moab changes, paper sorting by name