Since your washing this in, the treatment is applied to the inside as well as the outside. The problem is there is an icky waxy feeling against the skin. As someone else noted, it doesn't cover arms well, so you end up with a wet icky waxy feeling. It probably works as well as any other waterproofer I've tried, which is almost. It seems to wear off or wash away (even when hand washed in special soap) fairly quickly, maybe after 5 or so wearings of the garment. I wish somebody would make a real good waterproofer, I've tried 4 so far. From what I understand, you used to be able to buy some good stuff from clothes manufacturers or some dry cleaners, but the EPA stopped that.
Anyway, so far this is the best I've tried, at least it's easy to apply, you don't have to worry about missing a spot when spraying it on. Just get used to the creepy feeling.