I was lucky enough to convince my sis to give me a couple vintage shell chairs she saved from a remodeled dog track. So, I hunted around and found this article about restoring them to an almost original luster.
Quick and dirty summary of the process:
1. Spray the chairs with soapy water to cut down on the fiberglass dust factor when sanding.
2. Sand with sanding blocks starting with a medium grit and working down towards a fine grit (or if you want to get all Hyde on it, you can go as fine as you want).
3. Clean all fiberglass dust and water from the chair.
4. Apply Penetrol (yeah worst name ever for a product) with a rag that isn’t going to shed into the fibers, I used an old pillowcase. I ended up applying 3 coats due to the fact that they had been sitting outside and lost the clear coat on the seat and absorbed a ton of the Penetrol in the process.