It is common for the exterior timber cladding to develop stains around the fastening point.
Some timbers such as Western Red Cedar contain extractives that react with the fastener to cause staining. In many instances stainless steel or silicon bronze fixings are used, but the staining is still present.
Typically this does not affect the performance of the wood, but it does create an unsightly stain.
One solution is avoid exterior visible fasteners all together. Consider Abodo’s hidden fixing profile. By protecting the fastener from exposure to the elements the chance for oxidation and staining to occur is eliminated.