Although I haven't made a platinum print to date - I've been meaning to - but I would have to disagree with you last paragraph. I don't live in the Desert SW and have been printing kallitypes now for over a year in open sun, shade, overcast in all seasons without a problem.
As you know the kallitype requires an image that is closer to a latent image than a POP image very similar to a platinum print. James describes the kallitype image as being a "stage whisper" whereas he describes the platinum image as a "very faint image...This means that the image is simply undistinguished, a little more than latent, and a hint of what you are hoping for following the development stage." I think than my method of doing the bulk of the exposure in the shade would work best for platinum as the exposure time slows down in the shade allowing you the check on the print as it progresses. Incidentally James believes that the sun is the best exposure source although you would need a light-box for printing at night or in overcast. I disagree with the latter conclusion as mentioned above I have printed under completely overcast sky.
Thomas
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