Following instructions from Stachiw’s book, i got a tub of aluminium oxide and a tub of white tallow. Never heard of them before, just got them off ebay. Mixed a bit together to form a paste and polished with a cotton polshing mop. I was careful not to get the port too hot as just a few seconds polishing creates quite a bit of heat. I just went around once for a few seconds, then left it to cool. Took a while to do but in the end, all ok. Finished with cleaning with ordinary kitchen detergent.