Gonna put in a wash area for giving horses baths.  It will be in the middle of the barn and will have 3 walls.  What do you put on walls for waterproof besides treated lumber? thanks

Most I’ve seen are concrete block or poured concrete walls - but that’s pricey.

you could use tin siding.  I would probably try to do a foot to two feet of concrete to contain the water and direct it to the drain though.