well maybe you are talking about a different kind of plastic welding than i am famillar with. The plastic welder i use is like a sodering iron sortof with a hole through it that you feed the repair rod through after you heat up the substrate, and it "welds" the repair rod and substrate together, (or the two pieces of plastic together with the repair rod bewteen them for say a rip or tear in a bumber). And that would just be a lot of work, not hard but tedios, which is why i would just use the SEM repair material.
Please inform me if you have a different way of plastic welding 'cause i am genually interseted in always learning new techniques that work well, especially with plastic repair.