The present invention relates to tools and methods for welding, coating, repairing, or otherwise modifying the surface of a metal substrate. More specifically, embodiments of the present invention provide self-reacting friction stir welding tools which have an internal channel that allows filler materials to be added to the weld joint or otherwise friction stir processed area. Embodiments also provide friction stir tooling having upper and lower shoulders disposed at a fixed distance from one another and connected by a fixed split-pin. The tooling can be configured for automatically dispensing varying amounts of filler material into the tooling in response to variations in thickness of a substrate being processed.