OK Here is what I did. I didn't have access to any hi teck priming gelcoats or the like. I ground the fiberglass as smooth as I dared and skimmed on a layer of good quality auto body filler. I made a curved "shoe" for the air board sander that matched the curvature of the helmet. I used 40 grit to smooth everything down. This showed a few valleys so I filled those. I am in the process now of sanding everything with 80 grit and finer. I will follow with a few coats of high build 2 part sprayable autobody putty. Done it this way before - should be fine.
Thanks for all input.