My pony keeps a mask on 24/7 during the summer. If I don't, the flies attack his eyes and they get puffy, red, and weep. Then he ends up rubbing them on the rusted pipe fence which makes them even more puffy and irritated. I take it off when I get there in the evenings so he gets a 1-2 hour break each day before I put it back on before I leave. He's never had a problem with it. It's got felt around the edges so that keeps it from rubbing.
There on the tips of fair fresh flowers feedeth he; How joyous his neigh,
there in the midst of sacred pollen hidden, all hidden he; how joyous his neigh