> I have never applied oxalic acid "vapor" since it requires a respirator and it doesn't really work with a beard.

The respirators linked to below is certain to work with all but the longest of beards.
There are other full-face respirators out there, and "full face" are the ones that beard-wearing folks have to use.

https://www.jspsafety.com/products/cae602-941-100_powercap-active-ip-8hr-black
https://www.woodcraft.com/products/cleanair-hepa-with-ca3-papr-manila-s-r-o

There are also firefighter and military NBC masks that are full-face, but the days of true "military surplus" sold to the public are long gone, everything seems to go straight to local police forces.

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