Short answer: all epoxy resins gradually yellow as they age, mainly from UV light and heat. Products labeled "UV resistant" or "non-yellowing" slow it down but do not stop it forever. Yellowing in cured resin cannot be reversed; you can only sand and recoat the surface or keep the piece out of sunlight.
Common causes
Sunlight and UV: the biggest factor, especially outdoors or near windows.
Heat: including the heat from pouring too thick (see pouring too thick).
Old hardener: Part B often yellows in the bottle with age and air exposure. Yellow hardener gives a yellow tint from the start.
Wrong ratio: excess hardener can make yellowing worse.
Over-torching: too much flame on the surface.
How to slow it down
Choose a resin with UV stabilizers (often marketed with HALS/UV inhibitors).
Keep pieces out of direct sun; use a UV-resistant topcoat for outdoor or window pieces.
Store kits sealed, cool and dark, and use them within the shelf life on the label.
Measure exactly and avoid excess heat during curing.
White and pale pieces show yellowing most; colored or tinted work hides it.
FAQ
Can you fix yellowed resin?
Not in the resin itself. For a surface coat you can sand and apply a fresh clear coat, but yellowing through the full depth of a casting is permanent.
Is yellow hardener still usable?
Often it still cures, but it will tint the piece. Many makers use it only for dark or colored pours.
Does UV resin yellow more than epoxy?
It depends on the product. Both can yellow with sun exposure; check the brand's UV-resistance claims.
How long before resin turns yellow?
It varies from months in strong sunlight to years indoors away from windows.