Just speculating, but helium molecules are pretty small, so will be able to leak through tiny gaps where the paint/grease would not penetrate.

Cryogenic temperatures maybe?

> paint onto the outside of a vacuum flask, and have it be sucked into any gaps and solidify there.

Stuff like paint will tend to be very permeable- if it relies on a solvent drying out, then obviously its permeable enough to let the solvent through. Even if it does dry to a very solid state, there will be a very very long tail of evaporation of volatiles inside the substance.

> It seems they already make a product for that - vacuum grease.

Vacuum grease isn't perfect- it's not recommended for the high end of high vacuum or for UHV. Or for single digits of kelvin.

Another big factor is that if you're doing science, you probably want to go out of your way to eliminate contamination like that. Guessing about the possible chemical interactions of a secret proprietary substance under extremely low temperature and pressure is not a fun way to get your doctorate.

Yes, a huge factor is the outgassing of materials. While just solidifying and a long time after that, it gases off solvents and other small particles that you constantly have to pump away and that raise your pressure noticeably.

In cryogenic environments this is less of an issue because things are more sticky, but in a room temperature apparatus targeting ultra high vacuum this is troublesome or impossible.

Not sure if there's vacuum grease that can work at very low temperatures, the upper stage of the cryostat would go to 300 mK and the lower stage to 20 mK, at that temperature non-metallic compounds tend to get porous. There are materials that can work in there but to my knowledge nothing that was an easy solution, at least at the time.

If you're dealing with single molecule cracks and extreme vacuum, even vacuum grease is going to offgas and contaminate things.

Hydrogen is EXTREMELY small, the temps the poster is talking about are very very low, and XHV/UHV is extremely low pressure vacuum.

Like 'single molecules we don't want rattling around is a problem', 'more vacuum than outer space vacuum', and 'oxygen long ago turned into a solid' temperatures.

[https://en.wikipedia.org/wiki/Vacuum_grease]

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.

For a schlenk line in chemistry where you need 0.1 - 1 mbar, sure! But this is insane physics territory... Like other people said, grease isn't behaving like you think it would at millikelvin temperatures, or at pressures lower than those in the interstellar medium.

If you have a big aluminum chamber under ultra low pressure, guess what you're concentrating in there? Hydrogen and helium from the atmosphere that's seeping through your metal. Crazy stuff that's (sadly) not fixed by slapping PTFE paste or vacuum grease onto it.

There are some products but in systems with metal seals you don't use grease/sealant as as they off-gas various contaminating small molecules

https://www.tedpella.com/vacuum_html/High_Vacuum_Leak_Sealan...