For tar removal, there is a product called Removit by Dewitts Co. It is a very strong citrus based remover, similar but stronger than most citrus strippers.
For example, a customer needed roof tar removal when the house he purchased had tar all over the chimney. It was a mess.
All the excess tar must be scraped off using a chisel or scraper. Not removing any excess tar will make a large gooey mess when the stripper softens the old tar. Plus it will take more remover to clean the thick tar areas.