I'm afraid that the way you are building the parts, you are stuck with lumpy surfaces that need to be ground and bondo'd. The only way to get that super slick surface is to build a female mold with a polished surface and use gel coat as the surface of your part. There is nothing to be ashamed of using mold-less method - been done that way for decades and the overall body itch you will have for several days after grinding the part smooth is a wonderful badge of courage. I have done it innumerable times in my illustrious rodding career! In fact, Ed Roth built all of his wonderful creations that way - laid fiberglass over a plaster mold then ground and bondo'd the rough outer surface for a show painted finish. I commend your goal to make a headliner but that is one fiberglass task I would NOT try! You stand the chance of entombing yourself in hardened dripping resin for eternity!
Here are couple of photos of a multi purpose kick panel I made for my '36 Pontiac. It holds a speaker, the brake release, A/C outlet, a trunk/gas door remote actuators (just embedded the plastic donor housing in the fiberglass) and is a cover for the GM emergency brake. Ready to sand bondo and upholster.