I'm actually making a helmet too XD
Covering the helmet in paper mache, or something to make it more durable (I used paperclay) is a great idea.
I've found that after sanding the spackling paste down, I can strengthen it with gesso on top so it doesn't dent as easily. I used spray gesso (gesso in a spray can) to get it to look as smooth as possible and avoid any brush strokes.
Definitely put a few layers on there, sanding lightly in between each one.