Hey all, I've always cleaned my bongs with methylated spirits and rock salt and it works wonders, but I've noticed over several months of use a darkened tinge starts to slowly build up. If i touch it with a brush etc it comes straight off but there's areas around the percs in my g-spot its getting quite built up and I can't get into with a brush as they're sealed up. Everything else I've tried; methylated spirits and salt / bong cleaner / hot water and soap etc doesn't have any effect on this stuff and I have no idea what it is. Any one had any success getting it off with some solution??