For exterior: Should I wrap the vertical exterior plastic pipe (3/4 inch ) with the electric tape then install fiberglass tubular insulation over it to keep dirt off the tape.

Constant wattage cable cannot be used on plastic pipe unless it is filled with water at all times. Never spiral or wrap heating cable around pipes. Keep the heating cable straight along the pipe. In order to obtain even heat distribution, WarmlyYours recommends wrapping plastic pipe with aluminum foil before applying the heating cable. Never use more than ½“ of fiberglass or other non-flammable pipe insulation because this can cause the cable to overheat and cause a serious fire hazard or electrical shock.