I once got expanding foam on my hands many moons ago whilst repairing our little group dive boat , whose fibreglass hull had been breached whilst sat on its trailer (filled up with water - approx 3 tonne of it!) It was my first time using the stuff and I didn't know its perils. Once on my hands I reached for a cloth which unfortunately had oil and grease on it. Net result next day both of my hands were covered with black- hardened polyurethane. I spent the next two days slowly picking it off( thank god for sweat) Now whenever using it I wear three pairs of gloves (vinyl ) and a bin liner in case the valve snaps off and a copious supply of clean cloths and Isopropyl Alcohol and brake cleaner and very old clothes! Similar with Sikaflex -brake cleaner is excellent for obtaining a super smooth finish.