I’ve often seen a waxed capacitor used on older Fender lap steels, electric guitars and basses made from a company called Cornell Dublier. They made many types of electrical components and Leo Fender used the CD capacitors on many of his instruments till they were no longer available. Leo commonly used a .05 mfd and a .1 mfd for his tone controls in Broadcasters, No-Casters, Telecasters, Esquires and Stratocasters.

After the foil capacitor was made it was dipped in wax to keep moisture out and help eliminate the parts from absorbing moisture and changing value. Leo also used this approach when he designed his pickups. This was used to help eliminate moisture from being absorbed in the coil and vulcanized fibre flatwork.