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At the altitudes these are flying at (10km+), the air is bone dry. Unless moisture is encountered on the ascent, there is no need for conformal coating.

P.S. - I've done a fair number of these flights and can speak in more detail about this hobby on request. Great fun!

It amazes me that the balloon can last that long. That it's not losing it's loft over time.

Maybe they just put on a thin layer of conformal coating?

What about clear coat paint you would get from a hardware store just before launch?

fwiw You can get urethane and silicone conformal coating in a spray can and it's not that expensive.

Agreed, and your wife won't be upset when she discovers all the nailpolish is gone :)

I've always used clear nailpolish as a hacky version of conformal coating.