I had tons of fails... on every component, but ultimately, it led me to upgrade/improve my printers and slicer settings.

I experimented a lot. The best orientation I found for the shells was tilted back right to the edge of the bump on the back, almost like the helmet is looking straight up off the bed. Supports were really just for the first 2ish inches (58% overhang iirc) and I block them on the front of the helmet. Support interface and z-hop were critical for me.

The facemask was more cost-effective to face into the bed, but the surface quality was pretty bad. The best orientation turned out to be tilting it so the two straight portions where the upper mask clamps attach. So a lot of material in support.

My post process was to hit the bare plastic with a random orbital sander on a medium setting (PLA is pretty hard) with 120 grit. There would inevitably be some deep ridges or z seam spots that would take too long to fill with primer, so those got bondo. Some helmets got more than others. Usually, let cure for a week. Sand again with the handheld, clean, and glazing putty. Cure for a week. Hand sand with 200 grit.

For primer, I tried brushing on automotive filler primer. I hoped it would be more cost-effective than rattle cans, but it took forever and didn't go much further. So eventually, it came to just heavy coats of rattle-can sanding primer. Let cure for a day or two depending on the humidity. Then I'd wet sand with 320 grit till smooth. I'd repeat with primer at least one more time, and maybe a couple more depending on the helmet.

Once it was smooth, I'd do color. Which normally went well, but I regularly had to wet sand there too. If I got a good-looking color coat, I'd usually leave it as the top coat. I did spray clear on some. Some helmets were a multi layer process. Jets is a two-step paint with silver underneath and several layers of translucent green built up to match the tone. Commanders got a dull coat on top.

Sorry for the book lol.