What is causing this, and how do I fix it?  

RE: What is causing this, and how do I fix it?

First off if you have the option, switch to the smooth PEI Sheet, and use Windex to stop it from bonding to strongly. if its a must that you use the powder coated sheet try cleaning it first and possibly use Elmer's purple glue stick on it. The only other thing I might suggest, based on my experience and the picture you provided would be to adjust the tension on your extruder gears slightly. but mainly I would move to smooth PEI for a better bond. I own two powder coated and two smooth PEI sheets and the PEI has a much better hold on most filaments then the powder coated sheets do, but I have been able to force the powder coated sheets to success with elmer's purple glue stick.

Hope this helps

RE: What is causing this, and how do I fix it?

From my experience with an Ender printer, PETG adheres to PEI so well, it rips the film to pieces. The textured bed is supposed to be a practical print surface for PETG, so I want to get it working.

RE: What is causing this, and how do I fix it?

It is hard to tell from the photo how thick this is.  Are you sure it is not too low?  PETG is very sensitive to this and it would increase over adhesion.  

RE: What is causing this, and how do I fix it?

That blob appears to be a bit of scorched filament that was hanging to or around the nozzle and fell on the print.  Some people call these 'boogers' and they can cause all kinds of issues.  I'm in the habit of brushing off the nozzle area at the start of most prints, to prevent those.

RE: What is causing this, and how do I fix it?

@jsw

This is probably correct.  Check around your heatsink, do you have a leak?

RE: What is causing this, and how do I fix it?

Posted by: @watchm4ker

I'm trying to print the first batch of parts for the Lack Enclosure MK2, using a fresh roll of Signal White PETG. I've tuned the first layer so it's just low enough for good adhesion between the lines. But when I lay down the first layer, this happens, and has happened a few times now. 
What could be causing this, and how do I adjust it? I haven't had this problem with PLA.

I would clean nozzle and hotend properly and make sure that the nozzle seals well at the heatbreak. Also a high quality nozzle, e.g. E3D is recommended. I assume that the nozzle temperature is set correctly!?

RE: What is causing this, and how do I fix it?

@watchm4ker

Using Windex stops this from happening on the PEI sheets, just spray and wipe it off just as you would cleaning a window and your PETG will release almost exactly like PLA would from your PEI sheet. spray the sheet while its cool of course.

RE: What is causing this, and how do I fix it?

I understand, I had high hopes for my texture sheets as well, and while they do work, if you can get good adhesion, I have found them to not be very practical from an adhesion stand point.

I wish you luck,

Dan

RE: What is causing this, and how do I fix it?

@dan-p12

Same here. I just spent days fighting the textured sheet. I ultimately managed to get decent prints with PETG but it required slowing down speed drastically, fiddling with z levels, glue sticks etc. Just not worth it. Went back to the smooth sheet and every print came out perfectly. As others said, with windex cleaning and the application of glue stick or Layerneer PETG prints come of easily. 

RE: What is causing this, and how do I fix it?

Posted by: @karl-herbert

I would clean nozzle and hotend properly and make sure that the nozzle seals well at the heatbreak. Also a high quality nozzle, e.g. E3D is recommended. I assume that the nozzle temperature is set correctly!?

This is an i3 kit I assembled on Saturday, using the PETG gcode listed in the LACK enclosure MK2 blog post. The temperatures appeared to be the defaults for PETG. I never touched the nozzle except to brush off any residue after printing.