If you have access to a mouse sander I'd personally recommend going once over on the helmet raw before applying any filler. the friction should wear in a lot of that mess on the back section. I've had to do something similar to a few of my parts and that helped immensely before I applied any Bondo or filler primer.
Thank you for the advice! I was thinking a light sand and resin for the back side. Green stuff for the seams and then another sanding before using filler primer and a bondo/acetone mixture. I don't know if that's a good plan or not though.
 

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