Some friends of ours put in a doggie door on the sliding doors for the very reason you say, to keep the flies outdoors. It did cut them down to almost nothing. They make them especially for sliding doors and homes with sliding doors. Perhaps that will help.
They come in 4 sizes. Sold at Home Depot, Lowe's, etc. Also sold on pet store sites like Pet Smart or Petco, but at a much higher price for the same exact product. Easy to install and can't live without it, but I do have a passel of pets. From Chihuahua size to Great Dane . . . And a bunch of cats too.
I've been dealing with a bit of a pest problem in my garage lately, specifically with flies. It seems like no matter what I do and how much I clean, there are always ...
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Some friends of ours put in a doggie door on the sliding doors for the very reason you say, to keep the flies outdoors. It did cut them down to almost nothing. They make them especially for sliding doors and homes with sliding doors. Perhaps that will help.
Further to an answer I posted a few minutes ago:
https://holistichorse.com/health-care/keep-flies-away-with-water-and-pennies-2/
They come in 4 sizes. Sold at Home Depot, Lowe's, etc. Also sold on pet store sites like Pet Smart or Petco, but at a much higher price for the same exact product. Easy to install and can't live without it, but I do have a passel of pets. From Chihuahua size to Great Dane . . . And a bunch of cats too.
Here is a post I wrote that might help you https://chascrazycreations.com/irish-spring-soap-and-flies/