Well said, Jam.
If you've smoked.......well never mind, yeah resin build up on the bottom of the Carto. I also find that Cartos that start becoming harder to draw on after a couple of cleanings are destined for the garbage.
Cool Cartos seem to be best if you want 30-40 uses out of them. Wow Cartos tend to burn a hole through the wick after a couple of washings, you'll feel it on your lips as an isolated hot spot on the Carto.
I have 2 old Mocha Frappe Cool Cartos that have been cleaned 10 times or so and I don't think they will ever die. These 2 Cartos have probably had 90ml's put through them...it's crazy.
If I am batch cleaning, after I am done and the Cartos are dry I give them a test draw. Any that still have a hard draw, I remove the rubber stopper and toss in the garbage. I figure if they are still hard to draw on without juice, they will be even worse with juice. The odd thing is the hole always seems clear, but the draw performance is compromised.....why is that?
As well I only use thin juices which I think makes a big difference in Carto longevity.
Marine13, if a Carto produces 30% of the vapor it used to, is difficult to draw on, and has a burnt taste.....it's done. Even if you clean them, that burnt taste is generally a singe hole in the wick. A singe hole will then put heat directly on the polyfill and no matter how many cleanings will always alter the taste dramatically.