You will have to use extra body filler to get rid of that pep, polygonish look
basically, sand away with a mouse sander then i use medium or fine grit sanding sponge. The primer i used for the visors is called KILZ primer-sealer-stain blocker. Works really well, it patches up small scratches or rough spots where you sanded
Im not sure if its silicone safe so do a light coat of regular primer to the helmet