A guy from A1k.org has had success removing yellowing from a floppy cover from his A4000.
He used hydrogen peroxide (H2O2) in the following way:
Ingredients:
- 30% H2O2, water added to lower concentration to 20%
- Plastic container with lid.
- Protective gloves and glasses (always use this when working with H2O2!)
- Tongs to retrieve the Amiga part after bleaching
- His yellowed floppy cover:
He left it sitting in the sun on his balcony for 72 hours.
Here is a before and after shot:
before after