I’ve managed to make some progress.
With experimentation, I found a nozzle temperature of 250, combined with 100% cooling after the first layer looked the best out of all my other attempts. It still looked horrid when it came time to place the top solid layers, but it was the least horrid looking.
It then occurred to me that the only thing I hadn’t adjusted up to this point was my infill percentage. For PLA and ABS, I typically keep it at 20% fast honeycomb.
I use S3D, so I setup a print with four copies of the same hollow calibration pyramid model, each with a process set to use different infills percentages at 20, 40, 60, and 80 percent.
Again, the first layers and infill went pretty well. The 20% infill is the upper left square, and they increase going clockwise.
Here the top solid layers start to go down. The 20% square is really messed up to begin with.
I found that quality improved the more layers went down. I typically do four top layers, but for 20% that just isn’t enough with PETG.
The 40% square does better, but still has a visible blemish a couple mm in diameter. It’d probably do well at five layers. The 60% and 80% squares are pretty much identical.
So, there you go. More infill, more solid layers, or both.