Just to add to the discussion:
3M does not recommend air filtration at all when the actual air concentration of contaminants is unknown. Filters can be used (per 3M) if 1) the correct ones are selected for isocyanates, 2) the actual concentration is known to be less than 10 times the occupational exposure limit for isocyanates, 3) a documented change schedule is followed and 4) fit testing is performed.
Of course any DIY painting outside of a professional, tested spray booth always involves unknown and highly variable concentrations of contaminants.
My opinion for DIYs - forget filtration for isocyanates. Get a supplied-air system and be done with it. Or stay away from hardeners and urethanes altogether.
Ref: 3M Respirator Selection Guide