I've said it before on BF and I'll say it again: "Tecnu" The label used to say that it can be used as a barrier during activity, but now just says to use it after exposure, but before the rash starts. I'm very allergic to poison ivy and the ilk, but this stuff has saved me. I used to have to take steroids because it would get so bad, but I haven't gotten a bad rash since I've started using it.

My dad volunteers for the National Park Service doing trail maintenance and building. They buy it by the gallon for the rangers and volunteers.

You can probably wash off at the trailhead. Take the Tecnu with a gallon of water, and wash your legs and ankles right there. Once you get back home or to camp, you can do a better cleaning job to make sure you got it all.

I think most drugstores sell the stuff. Hope it helps.