What is that black AC sealant material?

67Dart273 said

We used to use a cork impregnated roll/ tape sealer in HVAC/R made FOR refer lines. I don't remember off hand the brand or name.

Here is one type

Cork Tape - VAPCO Company - Innovating HVACR for more than 65 years

Click to expand...

The reason you want insulated refrigerant lines to be as water / air tight as possible is that condensation can wick into and capillary up into and under the insulation. This is especially important with freezers because it will freeze, expand, and build up