Maybe I will get slammed for this, but I do not use any sealant.
It is a given that water will get in the conduit or will accumulate inside the conduit from condensation.
It is my belief that this is the reason for 300.5(D)(5)
300.5(D)(5) Listing. Cables and insulated conductors installed in enclosures or raceways in underground installations shall be listed for use in wet locations.