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First I wet the wood down, then spread sand over it and then scrubbed it, dipping the brush in a bucket of caustic soda + washing up liquid (and water of course). Found I got the best result when all this formed a loose porridgey goo, like facial scrub or something. You can see it in the left part of the second pic. Used a pressure washer to clean it off after a couple of minutes, working in sections.
What was the stuff you used? I have similar to do on mine, been looking at the ronseal stripper stuff to then seal again afterwards