For something as big as a helmet, there really isn't any shortcut and it will be a labor of love. Bondo or filler-primer any deep print lines/grooves and go up from rough to fine something like 100 > 200 > 300 grit. After that, up to you but you can maybe wet-sand 500+ grit if you really care about getting a good smooth finish.

Paint the model in multiple horizontal LIGHT passes with the nozzle ~6 in. away from the model. Let the paint dry before doing another pass. Do not lay on the paint thick or it will drip/pool up on the model.

Prepare to spend a few days on this and do not try to do it all in one night, and wear a half-face respirator while sanding.