Square tube reinforcement?

[cheapta]

I'm a Nascar Modified fan and love to look at construction details on those cars. One thing I've noticed are diamond-shaped reinforcement plates where two tubes are butt-welded together.Does anybody know how they do that? I mean I understand physically how it's done,I guess my question is-what happens to the weld bead that's already there? Do they grind that first? That would seem to me to weaken the weld before it's reinforced-not an ideal situation.