You /could/ try cleaning the carto in theory, but I find it's just better to use a new one.
If you have a carto tank, you can get them (even dual coils) dirt cheap. I think I purchased 25 (2.5 ohms) for $35. And, in case you didn't know, 2.5 ohms should sate all but the die hard vapers (like the vapers that used to straight-inhale Marlboro Red 100s).