My old horse use to get like that and it was combo of flies and dust. Allergies. You did good, keep the goop out and the fly mask will help a lot. You can also go get that clear eyes for horses (TSC) and use that. Also, some benadryl helps too. You can give up to 2 per 100 pounds. Try a low does of 3-4 25 mg tabs and see if that helps some.