To put it out just jump to prone a couple of times to put the fire out. Sometimes Napalm will catch your legs on fire or if you run past someone that is burning. If you don’t do something about it…
"Napalm B has a commonly quoted composition of 21% benzene, 33% gasoline […] and 46% polystyrene." This mixture is going to be totally insoluble in water, and common organic solvents are mostly flammable, so you’d only make the problem worse by trying to wash it off. A compatible fire extinguisher would be the safest bet.
Stop, Drop and Roll – How to put Napalm Fires out
In theory by using the napalm to start a backburn. But this is probably overkill. A backburn is similar to a burnout, but requires a slightly more sophisticated technique. Once a control line is established, firefighters may set a controlled blaze downwind of the main fire, just on the inside of the control line. Firefighters then push the new blaze back toward the main fire, burning up all …
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Reference:
https://www.usfa.fema.gov/
https://en.wikipedia.org/wiki/Fire_extinguisher