Personally, I would recommend that you premix the Vaseline & cotton balls. I like that method of fire starting. When the Vaseline is mixed well, it doesn't matter where the sparks hit the cotton, it will light. However, if you wait to mix the two, and it is 10F out when you need it, you might have a problem. Vaseline gets a little stiff when very cold. It will be difficult to work into the cotton so that it will burn effectively. When you most need it, your tinder pile from nature will not be at its best, possibly damp, so you will need the maximum burn time from that cotton to get your fire going. In addition, if your fingers are very cold and stiff, and you just got done trying to work Vaseline into a cotton ball, you will have a hard time trying to grip your fire starter with your cold, wet, and now greasy fingers. My recommendation is to pre-mix. Carry a Vaseline tube in addition if you want, or in a pinch, rub a coated cotton ball on your lips.