Has anyone used this as an external finish.

This is used indoors as a low suction key render before applying other coats of finished plaster. Think of it like an undercoat. I know many plasterers don't use it and use hardwall instead. It's lightweight on purpose so that plaster multi-finish can stick to it. Plaster is heavy and would run down on some surfaces, peel off or crack. Some trades use it as a filler for an old damaged plastered and painted wall where a piece of wall is missing, to fill the gaps after electrical duct removal and it's a cheap alternative to stucco. It's generally used to hide irregularities before the smooth coat. You still need to apply PVA to an old surface. I hope you didn't use it on the outside job, because that's what Tyrolean over cement based key is for.

I’ve never used it