RSPB nest boxes are made with thick FSC ® certified timber and will survive for many years, even without the use of preservatives.
If you do want to protect your nest box further, use a water-based preservative (preferably one that complies with standard EN71 part 3 as safe for children's toys). Such paint/preservative types are clearly marked on the container, and are available from DIY stores and hardware shops.
Preservatives should only be applied to the outside of the box. Apply in the winter, when there is a low likelihood of occupancy. This will allow time for any remaining volatile components to evaporate.
For more information about siting a nest box please see the Advice section of the main RSPB website.
