I’m currently welding some reinforcement plates to my car and before I weld the plates in place I’m drilling and re welding any broken spot welds but I’m having problems,the plates I can weld on fine no issues.
Is it steel though? some pretty funky alloys have been used in the last twenty years or so - maybe even longer.
If it was just sealant / wax / protective polyurethane then if you heat it with a gas torch, there will be some burnt residue.
Is it double skinned?...i've had problems with double skins where there is rust between the skins. The heat liberates oxygen from the rust (I think) and it all goes a bit rubbish.
You might have better luck drilling thro both layers and then using a copper backing plate (if there is access)...or drill the first layer and then clean the second from above with a flat spot weld drill before welding it.
I've had similar issues...on mine I cut it open, cleaned out the rust and then rewelded....but that was a heap of work!