Greetings, Bill, and thank you for sharing your experience of coating Pictorico with Pt/Pd sensitizer. It seems to take and hold these iron-based processes well. But I'm still a devotee of fine paper so, like you, I'm not very interested in printing on plastic, either, but there is something pleasantly subversive about printing a 19th Century process on a 21st Century material!
I thought it worth mentioning here in case someone is keen enough to follow it up with more testing. The coating unevenness we both experienced is more painfully evident because we are viewing the densities in transmittance - when normally lighting an image in reflectance, any unevenness tends to be 'buried' in the depths of the paper. It should be feasible to coat with cyanotype by total immersion - but I agree this gets highly uneconomic for Precious Metal sensitizers! Maybe I'll put this back on my agenda.
Having now dug out my notes of 6 years ago, I found that a Teflon (PTFE) rod makes an acceptable coating instrument for Pictorico - provided one can get it straight enough - smooth and clean with no friction. I also see that I made a New Chrysotype on Pictorico - which came out a neutral, slightly bluish black - quite different to the result on paper. Whatever the surface coating is (one manufacturer says "ceramic"), it is quite a different substrate to cellulose for making siderotypes, possibly due to a very different concentration of adsorbed water and the (presumed) absence of sizing agents. Best wishes,
Mike