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.
A third layer seals up the mix of screenings and bitumin rubber. This locks them in the middle. Without a layer underneath, they can tend to lack adhestion. Without a layer on top, they can tend to come off.