Eleanor: What papers were you using and did all of them show the same problem? Coincidentally I bought a new 3880 as a second dedicated piezography printer just before I saw your post. As you did, I installed the Epson carts and ran extensive tests in preparation for converting to the Cone inks, but I have not yet changed over. I printed on the papers which have been successful for piezography in my 1900: Epson Exhibition Fiber, Canson Platine, Ilford Gold Fiber Silk, Epson UPP Matte all printed perfectly in color and ABW. However, when I ran Cone Studio Type 5, my favorite paper which I planned to use in the 3880, I was horrified to see a large thumbprint-shape scar, filled with horizontal white lines, in the upper right corner one inch below the image top and 1/2 inch from the right edge. This appeared exactly the same on several sheets. I experimented by opening the Mac/Lightroom print dialog box and under Advanced Media Controls raised the paper thickness to 5 (3 is default for EEF). On the next Type 5 print the scar was lighter, so I raised the setting to 13 which turned out to be my lucky number: the print had no mark at all.
Later I called Epson and the tech suggested I set the platen gap to Wider. He did not know much about the paper thickness subject. In any case my 3880 now prints Type 5 OK with Epson ABW at 13 thickness and Wider gap, and I have decided to go ahead and purchase the refillable cart sets and chips in the hope that the Type 5 and K7 combo will work in the 3880. It has been flawless in my 1900, up to 13 x 19 sheets, and much superior to the many ABW test prints I have just run on the 3880. I plan to use different ink combos in the two printers.
I emailed the surface scarring as a tech support issue to Inkjetmall, asking what settings they used on their 3880s and whether they had ever seen surface marks, and I'll pass along their reply. (Their customer support has been excellent in my past experience.) I don't understand the paper thickness specs in the 3880 printer dialog box which comes up with .3 mm when you enter EEF. The label on the EEF box uses the term 13 Mil, but Google references say that is a measure of length or volume, so I got lost at that point. I also couldn't find a thickness spec for Type 5, but it looks and feels no thicker than EEF. It does feel softer. The instruction at the bottom of page 55 in the 3880 Users Manual says the paper thickness setting can affect print sharpness. If that's true the setting is an important issue, suggesting perhaps that too high may detract from sharpness, but the Manual's guidance is so vague it is no help at all.
I'll let you know the results when I have the K7 inks installed.
Lynn Noah