-
We used this on our cills and stonework.
It does work but its a messy old job. You trowel on a thick layer then wrap it in the paper covering. Leave that for 24 hours and in theory it should peel off with the old paint attached, in reality you need to do this several times depending on the thickness of paint. You also need to wash it all off and use a PH agent to neutralise the acid, so its gets everywhere.
Some kind of media blasting would be the most effective I think. Probably not the cheapest option but I think can be done pretty quickly.