home made flysprays, not so good?

For the first couple weeks I had Enzo I was using differnt store bought fly sprays without any issues. Then my BO told me about the fly spray she makes and how well it works. Its made with Larrys and white vinager, (I dont remeber the proportions) So I started using her fly spray on Enzo for a couple days since I had run out of my own.

One morning I noticed these black spots on him, more like gunk, I could get most of it off by brushing, but I was so confused as to what it was, I couldnt tell if it was a skin issue or if it was dirt of some kind. Then the next day I notcied more of it. Im thinking its from the homemade fly spray.

Has anyone else ever seen something similar on their horse? Maybe its from the vinager?

I had bought a bottle of Larrys concentrate and was going to make my BO's flyspray mixture, but now I dont want to since I think the vinager is causing this gunk. What can I mix in with the larry's stuff to make a flyspray that wont leave gunk? Would just water be alright?

Thanks!

not sure what you gunk it, could be overspraying the horse, cause the vinegar isnt doing it.

I use 1 cup each, bronco(or what ever fly spray you use) 1 cup apple cider vinegar, 1 cup pinesol, and 1/2 to 1 cup skin so soft.
This time of year, I learned the hard way, to use the expensive stuff alone while out on the trails.

is it flaky or just kind of flecks of wet dirt? if it's flakes, you could have irritated your horse's skin making it flake off. I accidentally did that to my pony last summer with a homemade spray. Just give it time and hold off on the fly spray, it'll be fine. Use a little less next time. Or is the fly spray leaving a greasy residue that attracts the dirt? I've noticed certain brands do that. And if I spray too close and too much, there will be a circle of greasy dirt residue where the fly spray was.

I have seen the gunk from store bought fly sprays. It is black and kind of looks like spots of mud had flung onto them. I think it's when their sweat mixes with the fly spray or something