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You need to have sufficient cathode material to work with. As the cathode wears, it becomes contaminated with the "used up" material. That's why depending on the thickness or amount of contamination can determine if you are successful or not.
A clean function is usually the mildest and will remove the least amount of material.
Unfortunately, there's no way of knowing beforehand and why some may shy away from the possibly of destroying an otherwise watchable tube.
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