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I'd try using a burn-off knife. When cold the bitumen might come off in chunks, when hot it might scrape off like butter, so I'd experiment with both. Then white spirit and wire wool. Make a quick jig with blocks screwed into a plank to hold the sides and one end of each block, then you have a stop that you can get some good force against when scraping and can switch blocks quickly and easily.
ffm
This is what they all look like- it's a (very) thin coating of bitumen rather than a chunky uneven layer.