I had been struggling unsuccessfully with Arches Platine and Bergger COT 320 and the new cyanotype process. Belatedly I found a reference to a speckling problem with Arches Platine and the new cyanotype process. Platine coated smoothly, and did not exhibit highlight staining with either the old or new cyanotype process, but I was unable to overcome this speckling (mottling) problem. Interestingly, I saw speckling in the lighter midtones with the old cyanotype process on Platine, while speckling showed in the darker midtones with the new process.
I bought 100 sheets of 28" x 34" Weston Diploma Parchment from Bostick and Sullivan in June 2007. Along with 250 sheets of 11" x 14". Weston Diploma Parchment is a hot pressed paper with an extremely smooth surface. It is a rather thin (47lb), unbuffered 100% rag paper. I tried coating the paper with a cheap foam brush, but the surface abrades easily. I instead rod coat it, 5ml of sensitizer for a 10" x 28" strip. I do not use Tween 20, as the paper greedily absorbs the sensitizer without it. The paper buckles after the fourth or fifth pass of the rod, so you have to work quickly.
Like Platinotype its wet strength sucks, to put it mildly. But that is fine for cyanotypes which does not require much washing to clear. The paper tears easily when wet, but regains its strength on drying. It seems the warm color of the paper fades a bit with processing.
Mike Ware asked me why I had struggled so much with his process, and it was simply that I refused to let go of Platine and try other papers. It was John Dugdale who told me I was making this too complicated and that when he started with cyanotypes he went to the art store and grabbed 50 different kinds of paper and tried them all and used the one that worked best. I mentally let go of the Platine paper and grabbed the Weston Diploma and that eliminated the speckling problem. Mike had suggested the addition of citric acid to overcome chemical fog for some papers, and it worked like a charm. I let the paper air dry in the dark about 1/2 hour, and try to use it in less than a couple hours. The sensitized paper starts turning green after a few hours, perhaps due to my tungsten working light. At this point it becomes fogged and unusable.
New cyanotype on Weston Diploma has a Dmax of 1.31 on exposure of 2m 20s in my UV box. It has a huge exposure scale of 28 steps on a 31 step tablet. The smooth surface holds tremendous detail.