I guess I am a little late, but for others who may read this thread: If you want to smooth out the underside of you body panels before finishing, you may want to use a product like Evercoat Feather Fill polyester primer. In addition to the texture of the fiberglass, there were resin drips and grinder marks on the underside of my body panels which I found very unsightly.
This is a sprayable two pot filler/primer that is sandable and fairly easy to apply. I used an inexpensive gravity fed HVLP gun to apply mine and was very pleased with the result. (Trust me, I am NO painter!!) There are other products like, Evercoat Slick Sand, but it was only available in quart cans which cost almost as much as a gallon of Feather Fill. And, as far as I could tell, the two products were identical. Although this product was claimed to be comapatible with all topcoat systems, be sure and check it out with what ever you top coat with. Others make similar products as well.
In the engine compartment, I used a high heat Duplicolor product in aluminium for reflectivity. Every where else, including the chassis pan, I used 3M Rubberized Undercoating (NOT the plain 3M Undercoating or other types of undercoating! These can be a mess and many aren't much more than black paint. The NAPA brand rubberized stuff never fully cured was awful.) I wanted a build up for sound and chip protection. If I had it to do over again, I would have taken it to Line-X and had their bedliner material sprayed on (it doesn't have to have the deep texture like a truck bed.) The reason I say Line-X is that at the time I was researching this, that was what all the competion auto audio guys were recommending. Since product development is so rapid, it would probably be a good idea to see what those guys are using when you are ready to do your car.
Lynn