Last year I added slatted bottom racks to all 3 of my hives and the bearding virtually disappeared overnight.
Yesterday, because we are in a dearth and I was afraid of robbing, I cut my hives entrances down by half after having them wide open since spring. Tonight at dusk I noticed the most bearding on all of my hives that I have seen since putting the slatted racks on last year. I am surprised what a difference in ventilation reducing the entrances by half made. I guess tomorrow I will crack the tops a bit to see if that helps with the air flow.