I just mentioned I rebuild them cause thats where you see the black gunk, on the inside of the cotton not the outside. I suggested it was from high VG but thats not really true it's that most companies add more flavour to high to high VG to get the same result. You can run a tank of unflavoured to clean the cotton too, would give the same results as a wet "dry burn" but may taste a little funky lol.