cleaning/shining shell chairs  

ideas
i've used water, soap & water, vinegar, wd-40, magic eraser, and plastic scratch remover & polish. i have used fine sandpaper to remove tape/glue residue, but that will likely remove some of the gel coat and plastic so it will not look perfectly even with the rest of the chair.

what about armour all or...
what about armour all or something like that to return the shine or is it better just to leave it?

I used
I've used a dozen different polishes. THe best one is called NuFinish. You put it on, let it dry, and then buff it out. Its nice because it retains a lot of the surface - but still shines nicely. If the gel-coat is totally gone, a matte finish with exposed fibers (you'd be itchy sitting in it) you can have it clear-coated at an auto body shop.
Found at your local hardware store.

..
armour all might make it shiny for a little while but i imagine it would wear off eventually.