If NEMA 4x or 6 (European IP 68 - submersible) is required, O-Rings or Seal Rings (ordered separately) should be used when mounting Strain Relief Fittings to a control panel, secured with a locking nut.  The Straight Strain Relief Fittings feature a molded O-Ring groove.  For these fittings, the Buna-N O-Ring is the best solution.  The Snap Elbows and some of the Conduit Fittings don't have a recess for an O-Ring.  In that case, the Polethylene Seal Ring would be the best choice, unless the hole can be bevelled to accomodate an O-Ring.