Keep in mind that:

1. The pipes usually rust from the inside out, so what looks like a simple hole might instead be the visible portion of a large network of virtual holes; and
   
2. the pipe is probably rusted paper-thin everywhere else. Trying to weld to rust is an exercise in the impossible; and
   
3. because everything's paper thin, it's weak as hell. The instant you start fondling the pipe, it will fracture at the rusty spot(s).