The timber we use is called Merbau. This is a tropical hardwood and is used in many outdoor applications because of its durability, stability, resistance to splitting and ability to cope with all environments.
You can preserve the rich natural appearance of your panel by using a good quality decking oil or outdoor furniture oil. Simply rub the oil over the timber with a lint free cloth.
Merbau timber naturally leeches its tannins when first exposed to rain or heavy dew. This can be flushed away by hosing the panel. Any stains that are on the metal letterbox surface should also wash away with water. Over time the timber, if untreated, will weather to a silvery grey colour. This will occur with all hardwood timbers left in the sun.