Can I use masking tape for 12V max insulation?
In-tact plastic insulating tape can certainly insulate 12 volts without a worry. You need to make sure, though, that the tape cannot slip along the wires if you make a two-ended jointed (most tape will easily roll over the glue), fall off (if you make a single-ended (hairpin) joint and that sharp points (eg of solder) cannot break through the tape. Abrasion can also be an issue. In all, there are enough possibilities that, alone, it's not a long term solution to keeping two voltages apart if failure is unacceptable (for example, if a short might cause a fire).