Kevin - Very good advice, but there are some related seam sealing subtleties that bear additional discussion.
I have water repellency augmented all of my windshirts which include: the Special Forces L4 Epic, Patagonia Houdini, Montane Featherlite, and Wild Things Lite.
For testing I used my home shower set to maximum force. I wore a windshirt in the shower with a long sleeve cotton T shirt underneath to determine if there was initial leak points that occured before the general fabric itself leaked. I then seam sealed those leak points, waited a day, and then tested again.
The first windshirt I augmented was a Patagonia Houdini. The general leak sequence pattern I experienced for all of my windshirts was 1) ++++++ the zipper, 2) ++++ the wash instructions and hang loop at the neck, 3) ++ the upper seams, and 5) + the material itself. The Houdini uses a silicone encapsulation similar to Epic and so I used Silnet on this windshirt. I did the zipper and all seams. The zipper experiment worked great but, I discovered two problems doing all of the seams: 1) the Silnet showed through the shear Houdini fabric and made the jacket seams look soiled and 2) the Silnet seams were uncomfortable to my skin when I didn't have a shirt on under the windshirt. All my subsequent windshirt augmentations focused only on the low effort/ high return zipper fabric and hang tag seams which were consistently the first places to leak on all of my windshirts that I tested.
The fabric zipper sealing augmentation technique that I used bears emphasis since I have never seen this technique in print. Completely close the zipper and turn the windshirt inside out. Use Silnet to completely cover the zipper seam, the zipper fabric, and the zipper teeth. The zipper fabric will permanently absorb the Silnet as apposed to just layering on top but Silnet will not permanently stick to the plastic zipper teeth. The zipper will open and close easily and any excess Silnet will just flake off the teeth in little granules. I used an old tooth brush to quickly remove the granules that flaked off on the first few initial openings and closings. The zipper fabric now keeps out water just like an expensive water resistant zipper.
I use Silnet but Seamseal should also work on non Epic fabric seams. I haven’t tried Seamseal on a zipper but if someone tries this experiment on an old windshirt, please report your test results. Also, if anyone else does a windshirt leak sequence test, please report if you discover a sequence different from what I experienced.