How to make something flammable that doesn't evaporate quickly?

Have a small acreage. My gravel driveway - about 200metres long - got overgrown with weeds during the extended wet season. I've poisoned the weeds, but need to burn them off before they mulch down and even more grow. Have tried petrol, metho, kero etc. to spray them with so can then burn, but the fuels evaporate too quickly and it is costing too much. Can anyone suggest how to make something, or mix something with whatever flammable liquid that I can spray the dead weeds with and burn without it evaporating so quickly? Am finding that I will spray an area of driveway about a metre wide and a metre long, then fuel evaporates before I get a chance to light it up and keep it burning enough to incinerate the heavy (dead and dried up) weeds And yes, I kinda need it to be sprayable so can use my backpack sprayer, otherwise it will take forever and I can't stay on my feet much due to serious back and knee injuries I got in my 50's (even carrying the backpack with 10litres causes agony!) Thanks. PS I tried mixing petrol with motor oil - it was a bit more effective, but still far to expensive for this disability pensioner.