The first layer is very important with PETG, as is the extrusion rate.
If your nozzle is too close to the bed for the first layer, you’ll get build up of PETG around the nozzle and it’ll be dragged with it for the rest of the print.
If you’re over extruding it’ll have a snowball effect and the PETG will keep building on the nozzle. It’ll eventually break away when it gets big enough and your nozzle will come around and hit it on the next pass, either making a mess or breaking the print away from the bed.
I found that a good first layer with PETG is the difference between a perfect print and a complete failure. Don’t squash it too much.
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