Those materials exist (see Apiezon vacuum grease and Lesker vacuum sealant). In less-demanding or desperate applications, they can work. But like similar last-

Those materials exist (see Apiezon vacuum grease and Lesker vacuum sealant). In less-demanding or desperate applications, they can work. But like similar last-ditch repairs (think automobile muffler tape or radiator sealant), they only occasionally last. Usually the best thing to do is to go actually fix the problem. The above advice assumes you're trying to maintain high vacuum (10^-5 torr or lower pressure). At low vacuum (10^-3 torr and above), that trickery can work wonders.