(see 6.4), the chemical conversion coating shall be applied nonelectrolytically by spray, brush, or immersion after all heat treatments and mechanical operations such as forming, perforating, machining, brazing, and welding have been completed (see 6.11 and 6.14). Assemblies containing nonaluminum parts that may be attacked, embrittled, or damaged in any way by the conversion coating process shall not be coated as assemblies unless the nonaluminum parts are masked.