Link: http://en.wikipedia.org/wiki/primer_usage

Anaerobic adhesives include retaining compounds, threadlockers, gasketing compounds, and thread sealers. Anaerobics cure in the absence of air and the presence of metal ions, primers ensure proper performance of the Anaerobic Machinery Adhesive when assembling inactive metal parts, which are low in metal ions. The following are inactive metals: Gold, Silver, Iconel, Magnetite steel, Black Oxide, Magnesium, Cadmium, Pure Aluminum, Zinc, Galvanized Steel, Stainless Steel, Titanium, Plated Parts, and Anodized Aluminum. The following are active metals: Iron, Plain Steel, Copper, Brass, Bronze, Nickel, Manganese, Monel, y Kovar. Primers are especially important for challenging assemblies of metal parts that are cold, have large gaps or deep threads.