The problem is compounded if the boards are used on exterior surfaces where they are exposed to strong sunlight and heat, which can melt the glue and wax. Coating these surfaces with an oil-based primer helps seal the wood's pores and prevents these substances from migrating outward. If you immediately think of fences and balconies when you think of weathered wood, think again. Many windowsills share the characteristics of weathered wood, especially if they haven't been properly cared for. They're horizontal surfaces where water can stand and where the sun beats down on them.
Wood deteriorates rapidly.
Don't be misled by the wood. Just because it's never been coated doesn't mean your prep job is going to be any easier. Weathered wood releases fibers very easily. The wood's cells are attacked by ultraviolet light and they deteriorate. In severe cases, the wood has a ridged appearance.