There are several previous threads on techniques for restoring the interior fiberglass panels on Fastbacks. I thought I'd share a couple of things that I've done that seem to work (for me anyway).
Easy Off oven cleaner (heavy duty - yellow can, etc) has been mentioned as the "go to" stripper for the interior fiberglass textured panels. I think the Easy Off approach works best if you have panels with the original paint. In my case anyway, a previous owner had painted everything with a thick coat of high gloss black enamel ...it was so thick that there was almost no visible grain in the textured panels. Per other threads and research on this and other forums, I applied Easy Off, wrapped them in plastic and let sit for a couple of days. Absolutely no effect whatsoever on the paint except that it looked really clean!
On a recommendation from a local automotive paint supply store I then tried SEM Urethane Bumper Stripper. That was the ticket...it works very fast...literally within a few minutes...and it peeled the top layer of paint off like nobody's business. I'm not sure how long a person would want to let it stay on the piece without causing damage but I limited the exposure to 15-20 minutes and then cleaned every thing off with a garden hose and scraper. Now we were down to the original factory paint.
Back to Step 1, I then applied Easy Off, wrapped the panels in Saran wrap and let them sit for about 24 hours. That more or less did the trick...it softened up the paint enough that with water and plastic dish scrubber most of the paint then came off. There was still paint present in the nooks and crannies of the texture that wouldn't come out no matter how much elbow grease I applied.
Knowing that the remaining paint was soft and wouldn't take much to come off I decided to finish the job with my blast cabinet. I loaded it with crushed glass - size "fine" and turned the air down to about 30 psi. The remaining paint blew off quickly with no damage at all to the texture. It ended up being a multi-day project but I'm very happy with the results.
For painting, I've become a big fan of epoxy primer and use it on virtually every part of the car that has paint. It seals surfaces very nicely and is essentially bullet proof...a little expensive and more difficult to learn to paint with than other products but well worth the effort. Where its particularly nice is on the smooth fiberglass panels...when those pieces originally came from the factory they were smooth as glass but over the years of being installed they get dings, scratches, rough areas, etc that jump right at you with fresh paint. The process I'm doing is to hit the smooth panels with a good coat of the epoxy, let it cure for a couple of days and then sand smooth with 400 grit. At that point (unless you have major defects that sand clear though and need body filler, more sanding etc) you apply the color coat (charcoal black in my case) and you end up with a beautiful result. For the textured panels, I also shoot a thin base coat of epoxy primer followed by the color. As others have mentioned, the color coat, especially the charcoal black doesn't seem to cover all that well without a dark base coat.
Not sure if this approach will work for others but, for my current project anyway, things have turned out great.