[Kirk Keyes]

IPA (iso-Propyl Alcohol) is not a hardener, but the other 9% might be something to be concerned about. Some denatured alcohols have some pretty strange ingredients in them.

There's no need for a denaturant in isopropyl alcohol (isopropanol) as it's one of the additives used to denature ethanol. Sometimes isopropanol when sold as "rubbing alcohol" has perfume, oils, surfactants in it, but those typically contain 70% isopropanol. (Ethanol is sometimes sold as "rubbing alcohol" as well.)

I see the product listed on Walgreens web site as "Walgreens Apothecary Isopropyl Alcohol 91%" - it very likely only contains isopropanol and water.