I sometimes use Swissgold (single-cup) filters. How do you clean these? Does it make sense to soak them in Urnex / Puly every now and then? I always rinse them under the tap first and then wash them in Fairy (aka Dreft).
A couple of days ago when I just rinsed the filter with hot water, I could feel the filter was still oily and indeed the coffee was rather bitter!