Posted by: @jareeny

Same issue here! I recently switched from MK4 to Core One and ran into almost identical problems using Prusament PETG with the stock 0.4mm HF nozzle — stringing, blobs, and incomplete prints. Interestingly, the same spool performs flawlessly on the MK4. I went down the same rabbit hole: replaced the nozzle, heater, and thermistor — no real improvement.

What finally worked for me was:

Switching to a regular 0.4mm brass nozzle (just like some of you suggested).

Lowering the temperature by 5–10°C from the default in PrusaSlicer.

And tweaking retraction settings slightly (extra 0.2mm).

Now I’m getting clean, reliable PETG prints again.

It does seem like the 0.4mm HF nozzle is very sensitive with PETG — maybe too much flow for standard print speeds and layer heights? Curious if anyone has optimized the HF profile specifically for PETG with better success — or if it's just better to avoid it for now.

I think I'm running into the exact same issues, both with 0.4HF and 0.6HF nozzles--trying to print lots of large things, I figured I'd go for a larger/HF nozzle, but the change in quality is killing me. Have you refined your presets any further?

I definitely have more stringing and little blobs with the stock HF nozzle compared to my Phaetus SiC 0.4 nozzle. Might be due to a different temperature inside of the nozzle or the 3-channels-HF design. I can lower the nozzle temp with the stock HF nozzle quite a lot without clogging, but when it starts clogging, it's quickly getting "dangerous".

It seems to me, that finding the sweetspot temperature is a lot trickier with the HF nozzle and even the same filament needs to be dialled in for each color of it. 

1. "Standard  preset": Prusament PETG preset or generic PETG preset? They are quite different..
2. "Filament was dried": there's "I dried my filament" and "my filament is definitely dry". When using my little eSun dryer, I have to use the highest temperature, even for pla. I put two of these little hygrometers next to the spool and the show about 48°C. Then I shortly open the box every now and then (3-8 hours) to let the moisture vent out and rotate the roll 1/3 rotation. I let it sit there, until both hygrometers show below 20%, preferably their minimum (10%). With some PETG rolls, it took the full 36 hours that I can set the timer of the dryer to. Only then I won't have any oozing at 235°C nozzle temp.
3. I have 40-60% humidity at 21-24°C room temperature here in Germany. I had some standard Geeetech PLA starting to ooze/bubbling/stringing after two weeks on the printer after drying. Which is why I put all my filament straight from the dryer into a sealed cereal drybox. Humidity starts to creep up after about 3 months and the silica gel has to be dried/swapped after 4-5 months. 

I've printed 4 rolls of black petg without issue and a roll of Prusament Ultra Marine PETG without issue once I got the printer fully adjusted.

Posted by: @brian-12

I've been using the Prusa HF nozzle with PETG without issue.  I also print Hatchbox filament and use the Prusament profile. 

Almost no stringing.  This is with black, and filament is dried and printed directly from a drybox.

Colored PETG does not print as nice and will have some nozzle buildup and light stringing.  Something about the pigment changes it's properties.  White is the worst.