The seal on our washing machine went black and mouldy and there was always a build up of sludge inside the seal, so I tried all the usual: high temperature wash, white vinegar wash etc etc but no real improvement. I was about to lay fro a new seal then read about using powered instead of liquid detergent. There wasn't an immediate improvement and after three months the door was good as new. So just switch to powder - if liquid detergent doesn't that to the door makes you wonder what it's doing to clothes in terms of leaving a deposit.