Yes, sandpaper will help a lot.  Use a fairly coarse one just to take off the main peaks and then a fine one.  Another method for finishing is to paint an area with past, allow to absorb and then run over it with the back of a spoon.

Yes to both comments above.

And it IS easier to prevent than to sand!  The next time, use smaller pieces of paper so folds don't form when they're going around curves.  And burnish each piece down firmly with the back of your fingernail or the bottom of a metal spoon.

Also, when you've got most of the lumps smoothed down, you can help to disguise minor imperfections by coating with several layers of acrylic gesso. Maybe sand a bit after a few coats if there are still some lumps, then add a few more coats.

That looks like it's going to be one great-looking helmet!

Sue

You've got too many lumps to depend on just the gesso.

Are you using a piece of wood under the sandpaper? If not, find a small block of wood. Mine is a 3.5" long piece of 1x2.  I just wrap the sandpaper around it and start sanding. Just fingers and sandpaper take a looooooooong time!

Starting with a coarse sandpaper and working to a finer type might be good.

Sue

A thin layer of pulped paper over the helmet would make it look better and stronger imo. There's a bit of work involved, but sanding can be very time consuming as well. I get fairly smooth surfaces by making a soft pulp, as much lump-less as possible. When it's semi-dry I go over it with a dampened flexible tool to smoothen it more. This way you can really cut back on sanding time. I guess you gotta put in the time regardless what method you choose, that's the bottom line. Nicely done helmet btw.