Hot paraffin wax dipping is an effective wood preservation technique. Once effectively treated this way, woodware, even unpainted can last up to twenty years. The difficulty is that the paraffin needs to be between 130C and 180C and the woodware immersed for about ten minutes. It is a hazardous operation and restricted due to the capital cost of the equipment to commercial beekeepers. Paraffin wax, often mixed with microcrystalline wax and/or tree rosin (pine gum) is heated in a vat to the desired temperature. The woodware is submerged in the molten wax for at least ten minutes, then removed to let cool. The wood needs to be in the vat for that length of time to ensure the wood itself gets hot through and through. This drives out the air and moisture in the wood, and when it is removed to cool, the molten wax is sucked in to fill the void. If you want you can paint the hot boxes and the paint is sucked in with the wax. It is incredibly effective protection for the wood, as it is the moisture in the wood that sustains mold and fungal growth. With the moisture replaced with wax, there is next to no chance of mold and fungal decay.
For the hobbyist, it is likely only an option if your club has a paraffin dipping vat, or you are friends with a commercial beekeeper who has the right gear. Why not offer to pay for some of the paraffin in return for using his equipment? Once your woodware has been done it will last for years.
*Suitable to be in contact with food products.