Best Finishing for Aluminum Microwave Waveguide

What is the best coating/finishing (if any) to achieve the most consistent microwave waveguide performance? The material is 6061-T6.

Standard extruded aluminum waveguide that has been manufactured by dip brazing does quite well with an Iridite or Alodine finish. I've even seen cast aluminum guide that's been finished this way and laying in storage for 30+ years with no degradation in electrical performance. There is no need to paint the exterior of the waveguide. In vacuum applications, the paint can be a detriment. External painting of the waveguide could provide the following:

1) Improved emissivity for thermal applications

2) "Pretty" equipment

3) Color tracking wg by vendor

Conversion coatings will work fine for certain frequencies but for other frequencies Silver is necessary to overcome insertion loss. Copper is usually used as a base for the Silver.