Adding Domeboro powder to cool cloth compresses can help dry weeping blisters. Some doctors recommend a topical or systemic cortisone-steroid for extreme discomfort. For serious cases with extensive blistering, consult a physician. Avoiding exposure, of course, is the wisest tactic. By learning to recognize the plant in different seasons and in different stages of growth, you can steer clear of it, or protect yourself by wearing gloves, long pants and long-sleeve shirts.
Some people pull up the wild parnsips in the evening, when exposure to sunlight is minimal.
If you do get the plant juice on your skin, the sooner you thoroughly wash the area, the less you will be affected. Over a decade ago, and newly arrived to Wisconsin where wild parsnip is common, I didn't believe my naturalist-friend who told me that it could cause blisters. Regarding myself as more of a botanist than she, and having never heard of such a danger, I scoffed. While she looked on dubiously, I picked a leaf, crushed it in my fingers and rubbed it on the underside of my forearm. As I'd expected, nothing happened all that day and I remember feeling a bit smug.