many many years ago I built a kit car, the kit used 90% donor parts from one make of car , some of these parts needed just a slight modification and some a good bit more to fit the kit car I was building,
one of these parts needing slight modification was the petrol tank, it couldn’t be used straight out of the donor car as the metal filer neck was 400mm too long and in the instruction manual it said 400mm was needed to be cut from the filler neck with a grinder,
the manual said even after the petrol tank was emptied of fuel, the tank would still contain petrol fumes and should be left to dry for a few days, and extreme care should be taken when cutting off the 400mm.
so once I emptied the tank of fuel I left the tank standing for a week while I got on with other things on the project, then one Sunday morning I took the grinder to the tank to cut off the 400mm, after about 20 seconds I heard a mighty bang and from the filler pipe was this blue flame that roared out a flame about one meter long, this flame and roar went on for a good minute if not longer
I know its not the liquid in the petrol that first catches fire but the fumes, and even after one week thinking the petrol fumes had all gone I was wrong, only afterwards I was told I should of filled the tank with water first before getting the grinder to the filler neck