A few things I’ve learned about hand warmers from using a Jon-E GI for years and the Zippo:
1. NEVER overfill it!! If you don’t have one of the Zippo or Jon-E filler cups, find out the exact amount the manufacturer says you should use. If you put too much fluid in it the fluid will expand and extinguish the catalytic element.
2. Never cheap out on lighter fluid. Get Zippo brand or Ronsonol for the Zippo, the Jon-E stuff for Jon-E.
3. Never touch the catalytic element material and keep it as far up into the burner cage as it will go without crushing it.
4. Use a butane lighter to “light” the burner element while the hand warmer is as upright as possible for a good 30 seconds to a minute. One of the long “grill” lighters works great for this.
5. Replace the cover and let the hand warmer sit upright for a few minutes BEFORE putting it in the bag so it gets maximum oxygen. IT WILL BE HOT, or it should be so be careful. I let mine sit for at least 5 minutes before putting them back in the bag.
6. If the hand warmer doesn’t get hot, try the same steps again but DON’T add any more fluid.
7. If it still doesn’t get hot, get a new burner. Believe or not this can make a HUGE difference if the hand warmer has a few years on it.
8. Don’t sweat the fluid. Naphtha is far from a major solvent and nothing tackle wise should come in contact with fluid anyway. But if it makes you feel any better, I use a Zippo lighters regularly to light my cigars, and hand warmers occasionally and never ruined anything or got skunked fishing on a day when I used them.
Obviously to make life simple you should light it at home before heading out or at the car. Trying to do it streamside will be a challenge at best.
Good luck and stay warm!