So I called Airmar who makes almost all the depth sounder transducers and asked about the myth of not using regular bottom paint on the xducer. The tech told me that the face of the transducer is made from polyurethane and paint solvents such as acetone "might" damage the poly covering.. Any water based antifoul paint is fine. No need to spend $ for something special. FYI there should be no degradation to sensitivity of the transducer when it is covered with ordinary bottom paint. As long as the coating is thin compared to the wavelength of sound (and it is) , it is transparent to sound. Those who report a loss of sensitivity when painted with ordinary paint are experiencing this loss for other reasons, not the paint, unless of course the paint solvent permanently damages the transducer. Actually I have painted the transducers on my boats for 60 years with ordinary bottom paint and never experienced any loss of sensitivity that could be traced to the paint. Those solvents evaporate so fast that I doubt if they damage the plastic enough to degrade the xducer. Richard P