If you want to fill a seam, so you can sand it down to make it "go away" then, Epoxy putty is going to be much tougher to sand than the styrene, so I would not recommend that.
Lightly sand your surface and use "Bondo" or like body filler. Unless your styrene is paper thin, and you pile on a lot of bondo, it will not affect the styrene. Sanding the styrene surface BEFORE adding the Bondo is critical. If you do not, the bondo will flake off on the edges when you try to sand if down very thin to feather the edge. (220 grit is good)
Important, the biggest mistake I see with fillers like Bondo, is too much being added at a time. Use care with how smoothly you apply it. It will save you a lot of sanding.
To smoothly apply it, use a piece of thin strene, or posterboard as a sort of scraper, and when you apply the bondo, use the card to smooth it, by holding the card at near perpendicular angle to the surface as you apply the bondo along your seam. DO NOT keep trying to work the bondo a any point after it starts to set up. Once it starts to set up, any messing with it before it fully hardens has a tendency to make more work for you sanding operation. If you have a big blob, remove it when the bondo is in it's "rubbery" phase, you can use a sure-form (A cheese grader like file) or a sharp knife to cut off blobs or excess material.
Only add enough too fill in the lowest spots. You can always add more. But if you add too much at a time, you just have a lot of sanding to do.
So, add a little, sand a little, add a little more to the low spots that the sanding reveals. Also, wet sanding is much more effective than dry sanding. Keep a cup of water and use wet/dry sandpaper, and keep the sanding area very wet, not damp, you need to wash away the sanded material, not make "Bondo mud".
The water will lubricate the sanding surface and cool the area being sanded. This cooling keeps the microscopic pieces of plastic from getting hot and sticking to the sandpaper. Your sand paper will not "wear-out" or load up as quickly, and your sanding will be far more effective.
Also, when your sanding, sand in opposing directions with each new grit (up/down then left/right) and don't skip grits. It take a lot longer to get 180 grit scratches out with 400 than it does with 220 . Typically I use 180-220-360-400-600.
Good luck, and show us your progress.
And remember, no where does it say you have to do this the first time, on your helmet. Find a scrap of material and test the bondo and the sanding on it so you have an idea how it works, Before, you slap it onto your helmet.
