The seal is a closed cell foam (non absorbent) with a self-adhesive backing. See also the cover plate, AlfaStop item # 1MW160.

Part Name: Fuel (Gas) Tank Sender (Sensor) Gauge Cover Plate Seal AlfaStop Product Code: 1RU205 Original Alfa Romeo Part No:  Model: Guilia 101 Series, Giulietta 750 & 101 Quantity Per Car: 1