Is it a die cast metal or styrene plastic?
I have used a plastic model putty called "Squadron" fast drying putty. The putty is available at most hobby shops. Score a groove in the plastic where you want the weld to be and using a large safety pin, work very small beads of the putty into the groove, pushing each successive bead againt the previous one to create the flowing effect of a real weld. You can practice the technique on the inside of a piece of plastic that can't be seen when the model is assembled. It takes a little time, but the effect is quite good after painting.
If it's soft enough, cut an angled groove with a sharp knife, fill with paint: L
Hot glue makes good fake welds on Gene Simmons armor. I assume it would also work on a model car.