About my Thoroid transformer...

[Voicecoil]

Reasons toroids are potted in a plastic case include reducing audible hum (potting compound locks the windings solid) and because they may look smarter! - important for HiFi. To do it properly you need a vacuum potting chamber to drive the resin inbetween the turns of wire. If the primary winding is on the outside you'll need some kind of insulation when potting in a metal can to preserve the breakdown specification - better to have the secondary on the outside in such designs.