Caution! Do not light the torch!
Recently I ordered a can of varnish to top off my arrow dip tube. During my conversation with Bob Burton of Whispering Wind Arrows, he shared this tip with me. Take the lid off the dip tube; hold a propane torch over the top; and then turn the knob and insert some propane into the tube, quickly replacing the lid. Since propane is heavier than air, it will displace the air in the top of the tube, keeping the sealer from drying out. Nitrogen will also work for those who have it.
Just a friendly reminder, DO NOT do this with the torch lit or bad things will happen!