Jaj, I went to google and then went to a site that shows how to use "deer fly patches". Maybe this would work for you. I don't know how you could put it on a dog though -it's sticky on both sides.

Jaj,
Saw an article to keep ‘biting flies’ (didn’t specify deer flies) off dog to use a 1:1 solution of Apple Cider Vinegar and water in a spray bottle and spray around dog’s face and ears. It was a natural remedy article.

Jaj,
This might work too. It's a tick repellant, but it might work for deer fly's too.
20 drops lemongrass essential oil
20 drops eucalyptus essential oil
4oz. Water
Just put it in a spray bottle shake & well, then spray it on the poor baby.

Safe for pets too

Hey Jaj,
Nita's right about the oils, you can also purchase a product called bugpellent (horse.com sells it) that is all natural. Uses some of the same oils plus a few more. I use it all the time on the horses, barn cat, the dog and me. Smells great, chases the flies away including the deer flies, gnats, and those tiny little no see ums (that's what we call them anyway). It works!!

Do they have eye goggles to protect dogs from loving owners? .... JOKE!!!

Thanks for all the suggestions! I’m going to check them out. Mabel sends kisses.