You can choose to not seal your urn if the ashes are placed inside using a sealed plastic bag (crematoriums usually deliver the remains to you in a plastic bag inside of a cardboard or plastic "temporary urn"); however, should you choose to seal the lid, it can easily be sealed with a good quality silicone sealant which can be found at any home depot or hardware store.
Simply apply a bead of sealant around the inside of the lid, and then securely fasten the lid on to the cremation urn. If you someday decide to remove the sealed lid, it is possible if done carefully by a fairly strong person. Simply twist the lid back and forth until it releases.
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