Waveguide damping

This is how I add damping to plastic waveguides to improve their performance. I use bitumin rubber which is mainly used for waterproofing applications. It has mass, adhesion and flexibility which make it work here also. 

Raw plastic waveguide:

To ensure good adhesion, the plastic surface is scratched up with coarse sand paper:

Then the bitumin rubber is diluted and applied as a primer:

When it dries, the surface is slightly rough and has the kind of texture that easily bonds to successive layers. Masking tape is applied.

After a few hours, a thick first layer is applied with a metal paint scraper and a brush.

Then a second layer includes fine screenings, which add greater mass. The screenings are cheaper than the bitumin rubber, ensuring that it goes further, but they also improve the performance.