Here are some Anti-Radar coatings .
TARNMATTE (camouflage mat ) was designed to be effective against the 9.7cm wavelength of H2S radar.It was made from 2cm thick sheets of synthetic rubber called BUNA that contained iron oxid powder. It was very flexible and used on KUGEL-SCHWIMMER SNORKELS etc. Its thickness dictated the frequency of radar radiation that was absorbed.
IG-JAUMANN ABSORBER made be IG FARBEN .This was far better at absorbing radar radiation but due to difficult maunfacturing it was not flexible and could only be made in flat sheets or in round forms ( it could not be made to cover the various shapes of a u-boats hull ) .It was made up of 7 layers of conductive material ( paper or plastic with carbon black ) seperated by layers of di-electric material ( rigid synthetic called IGELIT ,which is a polyvinylchloride foam that was 70 per cent air by volume ) . The total thickness was about 8CM thick. This material absorbed radiation between 2 and 50 CM .
ALBERICH . This was a textured synthetic rubber called OPPANOL. The idea behind this 4MM thick rubber was to cover the entire u-boat in this textured rubber. In reality this OPPANAL only reduced the sonar pulse by about 15 when the boat was at periscope depth.Absorption varied with depth,temp and salinity. The big problem with this system was that of addhesion. There was just no glue at the time that would keep the rubber panels in place. Over time wave action etc. made the rubber panels come lose and actually create more noise than a boat that did not have the coating. It was also found to decrease the speed of the boat by about 11/2 knots.