Do wooden beams have to be sanded before applying any products?

This depends on the type of wooden beams you have. If they are very old, uneven rustic beams, sanding may be difficult. If they are more modern, reproduction beams that have a smooth surface then it is always a good idea to sand structurally sound wood before applying any product. This removes any traces of previous product or contaminates, smooths out any defects and re-opens pores on the wood surface so it's ready to accept the wood stain or oil.