Buttermilk. The proteins in buttermilk can help drain fluid from poison ivy blisters. They can also soothe itchiness. DIY Health Remedy recommends mixing equal parts salt, buttermilk, and vinegar, and applying the homemade solution to the itchy, blistering rash. You may feel instant relief. If you find heat helps soothe the itchiness, you can also mix buttermilk with warm water and use this to wash the rash.