So, either increase the pressure in yhe flask and atomise the sealing fluid so it gets into the hole

OR:

Put the dewar in a chamber with the neck outside then lower the pressure and achieve the same.

Much like the tire slime put inside tires to seal punctures.

So, either increase the pressure in the flask and atomise the sealing fluid so it gets into the hole,

OR:

Put the dewar in a chamber with the neck outside then lower the pressure and achieve the same.

Much like the tire slime put inside tires to seal punctures.