so be it. maybe it's not the spark maybe it the glowing red hot 1000*+ metal from the weld that starts the fire.
try welding on the panels and let me know how it works out for you.
all i know is in the late 80's i was repairing a cavalier 2 door hit in the rocker panel lower 1/4 in front of the rear wheel.
i welded 4 heavy duty unispotter studs on the dented area about 1/8" apart. to attach a pull clamp to. when i reached for the 4 stud the inner 1/4 panel was filling up with smoke. i knocked the vent on the front of the 1/4 panel to blow the flames out. the flames got bigger after blowing on them. it took 2 fire extinguishers to put the flames out. the owner rustproofed with oil.
a few years later someone was welding a patch on a lower 1/4 after a crash. it was a overlap patch where both layers overlap by about 1". he started tack welding the patch on the corners. he didn'r get 2 welds done and the trunk was filled with smoke. car was also rustproofed with oil.
as i said earlier at out shop we get a car rustproofed with oil we WILL NOT WORK ON IT. this happens 1 or 2 times a year, maybe once every other year.

on a lot of modern cars where the panels are bonded like a ram door. no amount of oil in the world poured inside the door will prevent the rust forming in the lower flange. the reason why they rust there is because when the door is formed the metal is bare steel. where the inner and outer panels join is bonded then the flanges are stamped over the other while the glue is still wet. now there is bare metal exposed to the elements on the flange. ecoating cant get into the lapped seam on the outside and pouring oil inside of the door is worthless because there is a layer of cured adhesive preventing the oil from creeping into the flange.

also the biggest reason why rust forms on the wheel arch is because when the panels are joined there is bare metal. now they overlap the layers and there will be bare metal in the lapped flange. now when they ecoat the box sometimes a air pocket forms in the wheel arch area now you have a bigger bare metal area. also in the flange the outside seam is not sealed up allowing moisture to get into the seam. on some models a light layer of foam/ adhesive is put between the layers for sound control. this foam absorbes moisture that creates rust.
the best way to prevent rust from forming in this area is to apply a quality wax based, to allow it to flow into the seam when it gets warm, rust proofing that has corrosion inhibitors and seam sealing the open flange before driving the car or truck.
SORRY for this long winded response but i have been a bodyman since 1976. i have metal patched several hundred wheel arch rust repairs. i usually metal patch at least 4 VW 1/4 panel wheel arch bottom by the bumper rust repairs a MONTH. have been doing this for the last 5 years or so.