Magistretti plastic chair restoration - Help  

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It's been a while since I pulled out my Vicario or Gaudi chairs but from memory one is bigger than the other. ie. One is the dining / armchair setting for the selene chair & tables, the other is more of a lounge chair and larger of course.
As for sun bleaching you could try an all over light sanding and then a clear coat of lacquer, not unlike what is spray painted on cars.
They have a fibreglass component within the structure which makes them relatively easy to work with. I have only fully repainted these before not treated any sun damage.

Though Ball's suggestion may be
the ultimate solution, you might try buffing a less conspicuous area with a plastic polish such as Novus 2 or an automotive polishing compound first. I've used both to successfully remove "chalking" from gelcoat and to restore color and gloss on composite boat hulls and I believe these chairs have a similar pigmented polyester resin outer surface. In fact, if it works on a small area, you might then consider taking them to an automotive body repair specialist to save yourself some considerable effort.

Thanks for the suggestions.
I just read that Penetrol the paint additive works well and is much easier than polishing.

To respect the
integrity of the material, polishing would seem to be preferable to coating. Coatings can fail, and then need to be removed before further work is done, while polishing is simply returning the desired surface to the original material.
But, for a less-than-primo piece of furniture, or where speed and/or low cost are at the top of the list . . .