You should do monthly maintance of applying Noalox on the threads, give it a little time to work by turning the cap on the Provari back and forth, and then removing both the Noalox and the grime until the metal looks like new. An old tooth brush will assist with this.
Not only does it keep them working "buttery", but the black gritty matterial in the threads is actually the natural oxidation of the bare metal (not unlike rust) which could cause the threading to lock up if not attended to.
The oxidation buildup can also affect the conduction of the batteries, as the Provari depends upon the metal to metal contact of the battery cap to properly ground the electrical process. Any buildup of this oxidation can interupt this and cause the Provari to stop working.