honestly, the same mask you use for spraying isocyanate death paint. Its the VOC that are the problem with dope.
a larger concern when painting dope in a confined space is ventialtion and volitility. make sure you dont saturate the air with evaporated solvents, dope emits LOTS of them, dont use a residential fan to evacuate fumes unless its on the intake side of your ventilation flow, you really dont want to suck solvent through an electric motor that is not designed for it, sparks can do nasty things with solvent vapors,, like ignite a fire,,
a charcoal respirator designed for spraying is what you want, 3M makes them as do may other companies, lots of info here on the site, search for respirators