As an FYI - one of the first projects I did at the Dallas Railroad museum (Was Age of Steam - now called 'museum of the american railroad') was to remove (in texas (in august)) tar which had been put on an old pullman car that was 80+ feet long. They weigh about 100 tons and are made of heavy duty steel. The tar was put on to seal the cars and get an old car to last a bit longer, but shouldn't have been on there.
I took a garden hoe, bent the blade forward into a scraper and sharpened it. Then I mounted a blow torch on it and had it heat the tar as I scraped the 1/4" plus layer of that filth off by pushing the scraper forward. I could push it forward at a slow steady rate and hade the heat half on the blade and a bit in front. I then went over the entire car with a knotted cable wire wheel on a 1 hp grinder.
That was one of the nastest jobs I have ever done. But the car is happier today (the Glengyle - a 7 and 2 compartment car - national historic mechanical engineering landmark - made in 1910 - one of the first pullmans to get air conditioning - and that's where most people first experience AC, was on a railroad car).
I also have a needle scaler - but if you use a knotted cable wire wheel brush on a grinder, it will spin the normal layer of tar off in a hurry, but it will take off rust, and paint, and everything else as well!
