While I was on a volunteer fire department when it came time to train or in real life use an SCBA, self contained breathing apparatus, I would use Vaseline on my full beard to get a good seal. Messy but it works.
I did keep my beard trimmed short however so that helped but if your not opposed to the idea of shaving then the clean face is the best method.
Well, I guess being straightforward is akin to having an attitude, but. . .oh well.
SCBA is totally different from a protective mask. Generally, a protective mask intended to filter out particulates and chemical or biological contaminants. It does this by using the pressure generated by the wearer's inhalation to draw the contaminated air through filters. If you can't get a proper seal around the perimeter of the mask, this means that there's a chance that contaminated air will make it through the compromised seal, instead of going through the filter. Depending upon the nature of the contaminant. . .this could mean you die.
Now, another thing is buying surplus masks. Most military masks placed on the surplus market are rejects. There's a number of reasons they can be rejected from pinholes to microfractures in the mask to dryrot. Many of these deadlining causes can't be seen, easily and many dealers have sold, and will continue to sell, them completely ignorant of these facts.
Sadly, this means that if you want to prepare for a situation where you're going to enter (or leave) a contaminated site, you will have to spend some money to get a new, quality mask, learn how to use it, and obtain proper CBR-rated filters.
Attitude? So be it. But if you've ever seen the results of a nerve-agent, or read the symptoms, you'll understand why there is absolutely no compromise in protective gear. Even using Atropine or Pralidoxime chloride (aka 2-Pam Chloride to military types) isn't an option to most of us, and if you DO use it. . .there's a good chance that you're going to be incapacitated for quite awhile as both are ALSO poisons, albeit more temporary than permanent.
In addition to protective masks, any prepper who is considering a defense against NBC contaminants should also have protective wear since many nerve agents, etc. can kill by contact with the chemical or biological contaminant. Just protecting your lungs may not be enough.