The card stock pepakura is permanently part of the helmet. Usually it goes like this:
1. Card stock pepakura with supports to keep its shape
2. Coat of resin on the outside
3. Remove supports and coat the inside with resin
4. Fiberglass the inside using fiberglass cloth and fiberglass resin (2-3 coats) -OR-
Pour Rondo (Bondo and resin mixed 50/50) on the inside and slush it around instead of the fiberglass (multiple coats)
5. Bondo! (Car body filler) Apply in THIN coats to one section of the helmet at a time, and sand smooth. Repeat ad nauseam. Primer is helpful to spot imperfections by making everything a uniform color.
6. To get rid of the tiny holes and scratches, use spot/glazing/knifing putty.
7. Once it's smooth enough, primer and paint.
Here are some links.
1. Pepakura helmet from start to finish by member Thorssoli
http://protagonist4hire.blogspot.com/2012/01/prop-building-101-part-1-building.html
2. Part 1 of a twelve part video series tutorial for a Pepakura helmet
https://www.youtube.com/watch?v=QIHamhMdYYw
3. Lots of good pepakura videos
https://www.youtube.com/user/dghezzo/videos