Burnmarks  

Welcome to PETG

Clean off your nozzle and heater block regularly - you may have to raise the extruder high to get enough visibility.  It needs to be hot (230c) to clean well.  Be careful of the delicate wires on the left side.

Nozzle

Thanks for your reply, I tried already with a small steel brush but it is quite gooey down there and the brush is not a very precise tool. I just ordered a new nozzle, but I would like to clean the old nozzle thoroughly. Is there a trick to get it nice and shiny again? 

Nozzle changes - a whole new education - and won't fix your problem

I use needle nose pliers, a special nozzle cleaner I downloaded from the print library (holds two bent felt pads for chair leg floor scratch prevention).  If it's quite gooey, you have found your problem.  If the solution is change a nozzle every time the nozzle gets gooey from PETG, you will need a few thousand nozzles per year.

Nozzle Cleaner

Good point, so I understand this nozzle cleaner works, could you please share a link where I can buy it? Thx

Go to the print library and search "nozzle brush"

There's lots of variants on the print library. Nothing to buy except felt pads.  Click Prints above, then when the print library interface shows up, click on the magnifying glass (search) and search for "nozzle brush" - the ones that hole the felt pads work pretty well.  But once the good is well baked on, you may need the needle nose pliers ... get it off the heat block - it builds up a lot on the right side of the block sometimes.  It also builds up on the sides of the nozzle (where the hex is) .  Use a mirror, be careful not to burn your hands - the bed gets hot when you do this operation.

I clean my nozzles every day, sometimes before every print.  Some prints are more susceptible to build up - and need a clean start to finish without dropping any klingons onto your print.

Thanks a lot

I will get both! The part I am manufacturing at the moment is the top cover of a lamp foot, it needs to be perfect.