Many years ago before there were fly free supplements, or indeed any fly repellants, we used to use the plastic tags that were sold or cattle tied to a head collar. We also used to steep elder leaves and flowers in boiling water, wait for it to cool and used that as a repellant.
We have 2 ponies, a hairy new forest and a fine skinned arab x show pony who came equipped with fly rugs, fly masks and so-on. We soon discovered he disliked the fly rug - too hot and used to itch under it, and the mask (used to rub it off on anything he could, but without he got well and truly bitten all over, especially on his neck despite being smothered in various fly sprays and repellants, citronella shampoo etc.
As a last resort I bought a tub of fly free, and put both on it as the NF was attracting midges into his ears and also had flies biting him under his belly.
The show pony now doesn't need a rug or fly mask, however it made no difference whatsoever to the NF - the midges are repelled by power pahser in the tub which I wipe over his ears with the sponge that comes with it. I do dabble some PF under the NF and the show pony and round their tummy and undercarriage every other day or so as the flies do still try to land but you can see them land and take off without biting so something clearly doesn't smell good - but very few flies are actually biting them. The NF has a lot thicker and longer hair so I'm sure the flies land, can't smell the Fly Free as strongly and carry on and bite, he does get a few on his chest where I presume his tail doesn't quite reach!