Sorry for taking so long to post a reply. I have almost finished my headliner replacement in the saloon area. I use a matt finished FRP and mahogany battens to cover the seams. I used 3/4" blocks bonded to the fiberglass in locations as needed for screws in the battens, and to space the FRP off the hull to account for hardware and electrical. Seams for the FRP can also be covered with plastic strips made for the FRP.
The FRP comes in several colors and finishes. I choose matt white, but the most popular finish is the ice white, a textured rough finish.
Here is a link to Home Depot that has the FRP Ice finish. I had to special order the Matt finish from my local hardware and lumber store.
4 ft. x 8 ft. White .090 FRP Wall Board-MFTF12IXA480009600 at The Home Depot
I will try to get some pictures posted up in the next week or so. I also used mahogany 3/4 X 1/8 strips with rounded corners to cover the 3/4 welting used on the original liner. I use a mahogany 1 1/2 X 3/16 X 8' strips for my battens.
Summation of the project
If I had to do it all over again, I would just remove the headliner, put a quick sand job on the fiberglass, and paint it matt white. Yes you can see the hardware, and may have to put a few electrical ties to secure the wiring, but I think it would be an easier way to go. After all, I have a 1971 boat, and its only worth so much.
Making patterns, placing the blocks, etc, took loads of time, effort, and two people for most of the project. The sand and paint is a once person job for the most part.
Cons to sanding and painting, is the mess, using the correct equipment for sanding the fiberglass. I rather suit up, wear a hat, respirator, and vacuum sander, then have to do all of the measuring, cutting, and bonding blocks.
Pros of FRP headliner it looks better then sanding and painting. So, it's your call on how much work you want to spend on the headliner.
Hope this helped you
I'll be back soon with pics of my FRP headliner.