
Brake fluid?
Regular DOT3 brake fluid is quite harmful to paint especially if it’s unnoticed and won’t be cleaned off for several hours. You can pump the stuff through rubber brake lines no problem, so I imagine a balloon would be able to survive it.
I addressed that in another comment here. The long and short of it (very long, as it happens) is that the volume you’d need is still the same. So your elongated balloon would have to be well beyond what most people would consider to be ridiculously tall. 325.5 meters tall, in fact, given the 0.75 meter diameter I assumed to start with. I figure most people could probably stand in a 0.75m circle provided they didn’t wave their arms around a bunch.