In Testing we have discovered a problem that Photoprint has with producing smooth gradients (called Fountain Fills within Corel Draw).
We especially notice this in lighter gradient areas or colours.
One thing that helps this is to use Error Diffusion or SO diffusion screening, this will take a lot longer to process your files although will improve results.
The photoprint update i last mentioned is now out and not referred to as SP3, but as Photoprint 6v2
The problem has improved although the release notes mention radial gradients being improved or resolved. Linear gradients are better although the problem still exists. I reccomend updating to the latest build of the rip and trying “Error Diffusion 2” screening as this will also improve the problem.