I keep those butterfly sharpeners in my pack, and in my car. But just for luck I always carry a DMT diamond card in my wallet. It’s a metal plate, the size of a credit card, with one face impregnated with diamonds. I’ve occasionally gotten stuck on some job that dulled the knives I was working with, when I didn’t have the time to go get my proper sharpening tools. Fine. Out comes the wallet card, and I can restore an adequate working edge in a jiffy.