I don't use the condom method. How I fill and top off is below. I use thin clear juices and can get 7 days use out of each carto (which for me is 14 days using it 1/2 the time since I vape 510 1/2 the time and carry both models). I use PG w/ 20% or less VG, and Vapor4Life WOW juices (which are also about the same consistency). I avoid all juices that drop flavoring sediment (cuts short the life of a carto) and these are usually dark or opaque or cloudy juices. I also only use manuals - harder to damage or kill with juice leaks, easier to get a good hit.
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How I fill a cartomizer (Vapor4Life/Bloog/Volt for KR8 or Boge 510 for 510 which are all soft-cap w/ no inner seals and defined central air channel)
I use thin (20% or less VG) liquid or Vapor4Life WOW liquids, preferably clear liquids w/ no sediment
Filling a blank carto - I pop off the soft cap (unbent paperclip carefully in the mouth hole, not catching any wires or stuffing!). I hold some tissue against the threaded end hole. I tilt the carto and slowly drip 18 drops (per ml of carto capacity and these are 1ml cartos) onto the inner wall of the carto so it slides down into the stuffing while I rotate the carto. Then I wait 15 seconds or so then I drip however many more drops the stuffing wants to soak up.
Cleanup: Then I do clean-up. If it is leaking, I clean that up. Then I tilt the open end down into the tissue and puff a couple of times into the threaded end to clear any juice out of the central air channel (I want to see light through there). With the carto still tilted with open end down I gently touch a bit of tissue to the stuffing (otherwise that liquid will be gurgling or spitting in my mouth). (And instead, if I really want to avoid leaks I touch the soft bit of tissue to the stuffing while blowing out the air channel.)
Settle: Then since this was a fresh fill of a blank, I now let it settle. I lay the carto on different sides for 15-20 minutes. Then it should be ready to vape. If you use thicker juices than me, then you may need to let it settle longer.
As I am vaping - if the vapor gets a bit light or a bit dry or tastes bit off or the carto is getting unusually warm, I will take the carto off the battery, open it, add 6-8 drops or whatever it seems to want (I will either hold a bit of tissue against the threaded end hole just in case, or skimp on how many drops I add to make sure I avoid drips if I have no tissue). I check to make sure the threaded end is dry, give it a couple of puffs into the threaded end to make sure the air channel is clear, and continue to vape.
Note - it is easy for me to "open" my main cartos for adding juice because I replace the white end cap with a plastic or delrin drip tip (I get them at LiteCigUSA.net). So I can easily pull that out with my fingers when I add juice and it keeps my lips cool if the carto is hot.