[antonye]

The UV protection is in the resin, which is the clear coating of the part.

A cheap resin will have little or no UV protection and it will turn milky or yellow over time as the resin gets broken down by the UV rays. Keeping it out of the sun for long periods will obviously minimise this.

More expensive resins have UV protection in them, so they will not break down and need no special treatment.

How can you tell? Ask when you purchase, or purchase from a reputable supplier.