The three causes
| Cause | What it does |
|---|---|
| Ultraviolet light | The main driver. Sunlight through an uncoated window will visibly yellow a white sculpt or a box window in a single Indian summer. |
| Heat | Accelerates everything else. A shelf on a sun-facing wall, or above a device that runs warm, ages a figure faster than the same figure across the room. |
| Plasticiser migration | The softener in PVC moves to the surface over years, causing both tackiness and a yellow cast. Slowed by lower temperatures; not stoppable. |
What actually prevents it
- No direct sunlight, ever. This single change does more than everything else combined. A figure two metres from a bright window and out of the sun beam is fine; one on the sill is not.
- UV-filtering film on the window of the room, or a cabinet with UV-filtering glass. Film is cheap and works on the whole collection at once.
- LED lighting only, and mounted with an air gap. Halogen and incandescent bulbs put out heat as well as light, and a warm spot inside a closed cabinet is the worst of both causes.
- Keep ambient temperature down where practical. In a room that regularly exceeds 35–40 °C, an interior wall away from the window matters more than any accessory.
- For boxes: acid-free PET protectors, not PVC. A PVC protector can off-gas and haze the very window it is protecting.
Why you cannot fix it afterwards
Yellowing on a painted figure is not surface dirt — it is a change in the plastic and, for boxes, in the acetate. Nothing you wipe on will reverse it.
The technique people find when they search for a fix is "retrobright": hydrogen peroxide plus UV exposure, used to whiten yellowed retro computer cases. It works on bare ABS. It must NOT be used on an anime figure or a Funko: the peroxide and the UV attack the paint and the tampo-printed face, so the likely outcome is a figure that is whiter and also ruined.
The same applies to bleach, and to abrasive polishes — those remove the yellowed layer along with the paint on top of it.
What to do with an already-yellowed piece
Accept it and stop the progression, which is genuinely worth doing because yellowing compounds. Move the piece out of light and heat and it will stay roughly as it is rather than continuing to darken.
For a boxed Funko where only the window has yellowed, the figure inside is usually unaffected — the acetate took the UV. If display matters more than resale, taking it out solves the visual problem entirely.