Why should softwood not be sanded before gluing?

Sanding softwood before gluing is not recommended primarily because it can create dust that clogs the wood’s pores. When the pores are filled with dust, the adhesive cannot penetrate effectively into the wood fibers, which can lead to a weaker bond between the surfaces being glued. An effective adhesive bond requires that the glue has direct access to the wood's fibers, allowing for optimal adhesion. Therefore, keeping the surface slightly rough helps ensure that the glue can enter the wood more effectively.

Smoothing the surface, while seeming beneficial, can hinder rather than help the bonding process. It’s important for gluing operations to maintain an adequate surface texture to maximize the adhesive's ability to adhere to the material.