I don't clean cartos. They are hard to dry too especially in some climates.
I do keep them damp with appropriate juice (which for me is 20% or less VG light clear juices that leave little or no flavoring sediment - those give me the best decent life out of each carto).
This is how I fill blanks - the difference with just topping off would be only the number of drops it wants and it won't have to settle because it will still be damp with the previous juice (I never vape a carto dry).
If your end caps won't come off, then you chose a rebrander who uses hard caps or glued on caps. In that case you could try to use a carto condom to push in juice. And BTW if you have auto batteries, no matter how you add juice you have to make sure the carto will not leak at all on your battery. Auto batteries can be damaged or killed if juice gets in the hole in their threaded end.
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How I fill Vapor4Life KR8 cartos and Boge 510 cartos
Pop out the soft cap with an unbent paperclip in the mouth hole, very careful not to catch stuffing or the wire loop.
Shake and open the eliquid
Hold a bit of tissue on the threaded end hole, tilt the carto, and add 18 drops per ml of capacity into the stuffing (Boges and most KR8 cartos are about 1ml).
Wait 15 seconds or so then see how many more drops the carto stuffing will soak in easily.
Clean up the threaded end, tilt open end down into tissue, puff into threaded end to clear air channel (Boges and the KR8 cartos I use have a defined center air channel and I want to see light through that)
Sop up any loose juice above the stuffing
Cap and let it settle - I lay it on different sides for 15-20 minutes to make sure every bit of stuffing gets damp so I won't singe the carto. if you use thicker juice than me (I use 20% Vg or less) then you may need to wait much longer.
While vaping, if the vapor gets a bit light or a bit dry or the carto is getting hotter than normal, I take the carto off the battery and add eliquid. I sometimes use a plastic or delrin drip tip as a mouthpiece on Boge and KR8 cartos (not to drip into since the juice would head for the central air channel)
