Everyone that uses clamp tops has the odd shamefully black one - usually I clean them with petrol or Cif and a scrubbing brush (unpleasant and tiring).
Yesterday I heated up a 200l drum of water with two cubies of glyc in it, to about 70-75C. I had a pump to stir the lot up.
Anyway, just playing the pump outlet over the black surface of the drums I could see the mank starting to clear, and just a quick pass with a washing up brush and it fell off. I was amazed. Must be a combination of the heat and the glyc. Now my three mankiest drums are the three cleanest and it only took five minutes per drum. Even the fiddly bits (around handles and the lip at the top) were easy to clean this way. I've put the clamps in to soak overnight.
M67 heater in a plastic drum was a bit tenuous though, needed supports to stop it tipping.
