I never got a mere rash. I would get sheets of merging smaller (BB sized) blisters, up to dime sized. Calamine lotion is probably the most recommended, and for me, at least, least effective remedy.
What worked. Hold affected part underneath flowing warm water. Slowly increase heat until it feels GOOD (Like intense scratching would, only better) --it gets pretty hot by that point. At that point the pores of the blisters have opened and they are draining oils which ideally go down the drain, because (in my case anyway), those oils would only cause more blisters anywhere unaffected they landed. After the water is so hot as to be less comfortable, and the effect of having the itch scratched subsided, I'd shut the water off, and pat the affected area dry with paper towels (disposed of like they were contaminated with the plague).
Now for the unpleasant part.
Irrigate the affected area with rubbing alcohol (Isopropanol 50% or better--- NO SMOKING or open flames-- it will burn). Even without flames, the sensation is similar to being on fire, but it passes. This will sting like hell for a bit. And then the blisters start drying out. Wrap area lightly in roller gauze (at least for me, that's often how large the affected area). Two or three treatments, the blisters dry up, and the dead skin can be peeled away.
Any clothes/fabric or surfaces that come into contact with the blisters/seepage therefrom should be laundered as if they are oil contaminated. Needless to say, wash them separately from the rest of the laundry, and if you are sensitive to the poison Ivy/oak/sumac family, wear gloves.
As a youngster, up into college, I could get the blisters from contact with the plant, the stems in winter, the roots, animals and fabrics which had been in contact with the plants, even other people who had the rash.
My mother could catch it from the laundry, so we had to be especially careful to keep out of it for her benefit as well.
One other thing, even if you don't generally have a reaction or much of one, if you are pulling the plants (well, digging them) up, NEVER, EVER BURN Poison Ivy/Oak/Sumac. The oils will be carried in the smoke, and people can not only catch it from the smoke, but inhalation thereof can cause serious respiratory problems (a childhood friend was hospitalized after walking through the smoke from a neighbor's leaf pile which contained Poison Ivy plants, with severe respiratory complications--it nearly killed him.)
