My recollection is that the piezography inks were initially matte only. Then at some stage they started being sold as also suitable for gloss, although it's the same inks for matte and gloss, except perhaps for the darkest ink. I know that piezography gloss is meant to have the gloss overcoat, but from what I've seen of the before and after, it's as if the overcoat is covering up the difficulties that the inks have with gloss. And from what I've also seen, the cover up is sometimes less that perfect. My initial question was why piezography on gloss needs this overcoat and Epson inks don't seem to, or at least not to anything like the same extent.
Correct, the original Piezography was matte only.
However, it wasn't that "at some stage they started being sold as also suitable for glossy," with the GO just thrown in to cover up the problems. The K7 glossy, like Epson's OEM inks, uses a different 'photo black' ink from 'matte black.' And just like the OEM inks, you have to switch from one to the other, depending upon the media type you're using. Jon Cone's original 'MPS' system - his term to describe the K7 inksets applicable to both glossy and matte - covered only two of his five inksets. Later, he formulated versions for all five. You can today buy versions of Piezography that are matte-only, glossy-only, or both.
As noted above, the GO used in Piezography is an essential, integrated, required part of the K7 glossy process. Ink reflectivity, dMax, linearity - they are all significantly affected by the subsequent GO layer. The Piezography curve was created fully expecting that second half of the equation to be in place. It is not at all analogous to a gloss overcoat being applied to an already finished print.
I can't speak for all five of the Piezography inksets, but I've made hundreds of glossy prints with the
Warm Neutral and
Selenium inksets; and many more that are split-toned. In a very few instances - perhaps 2-3% - a particular image/media combination has required a second application of GO. But that is very rare. Ultimately, all are perfectly devoid of bronzing or gloss differential.