Yes, I think I got the same helmet that he is talking about. I got it on ebay a few months ago. The person selling them does not show you the inside of the helmet in the auction. Their is no way that someone could wear one of these. I will see if I can post some pics later.
I would still use sandpapper. (not a rotary tool or any other machine.. manually) Just to get the roughness out of it. (there is no way you'll be able to sandpaper all the way through and weaken the helmet)
Then perhaps use the bedliner or any other filler (bondo/gelcoat?)
Oh.. and wear gloves... glasfibre splinters hurt like hell and you can't get 'em out..
There is also a spray on Plasti-Dip. Go to the local hardware store. I got a can at Ace Hardware in town. I used it to spray onto my pair of gauntlets, because they to have fiberglass on the inside. It is made by the same company that makes the canned plasti-dip that you can use to dip tools into to replace the plastic grips.
dont toss old ones, if you want, ill work on em, I have suddenly been sparked by everyone around that iam an artistic genious, Ill buy the "broken" or "poorly done" ones from you (preffered cheap) put a crapload of work into em, and if you want them back afterwards, ill give em back for the same price, no charge for work. I liked makin my helmet and I want to get better, but finding more helmets is gettin difficult right now, and shipping killed me last time.