Doing my work on the 3D printed helmet. Printed from PETG. Had a tip from a fellow member in my Star Wars squad. That person used a non-shrinking wood filler, putty I call it, to fill in the print lines. The sanding of the filler goes very fast. Here are images of how it is working out.

I am using Elmer's brand interior-exterior wood filler. The label states non-shrink. This stuff has no smell to it. I was going to use bondo but I was warned that it is very smelly and heavy on the toxic side. It is winter here in the Midwest so I cannot do any of this outside in the fresh air. If you are stuck indoors, particularly if you do not want the house, dorm room or apartment to stink then the wood filler might be what you need. Yes it looks like baby poop.

Here is a shot of part of the helmet, showing the print lines and separation spot from the printer bed. The next image will be of the same area with filler added.

Not finished, just the filler buttered on with either your finger, popsicle stick or putty knife.

You can see that the putty covers well, and if you are on a budget, this stuff is cheap.

I got a sanding sponge, 120 or medium grit. I would say anything coarser than this would be too much. I went over the entire helmet once covered in putty. You have to wait until the stuff dries, but it dries fast. You will generate a TON of dust so wear a mask. Protect work surfaces as well and I would do this away from electrical devices like cell phones and computers, due to the dust. I used light pressure as well. To much pressure and you take the putty off and start seeing print lines again.

These shots are after sanding. You can see the surface looks smoother. It is not perfect. Looking the parts over and turning it to look along the surface in a bright light you should start seeing low spots, missed areas or places you sanded too much off. The end idea is for a smooth, line free surface. I circled the low and missed spots with a pencil. These are going to be filled in with more putty. A trick is to dip your finger tip into water and gently rub the fresh putty, smoothing it out on the print surface. If you just leave the putty on to dry and not smooth it then you are going to have to put more time into sanding all those lumps down.

I also noticed that if you hold the part up to a strong light you can see the missed spots. This will not work for all the places that need more putty but might help you all the same.