From someone who replaces body panels for a living, I rarely use a spot weld cutter. I do own the Blair Rotabroach and others. I use them when I need to disassemble and then reattach the same panels. They are designed to save the top and bottom panel. And the little button left behind needs to be ground off, meaning more work.
When replacing rusty junk like trunkfloors and old 1/4's etc, there is no need to drill anything. The fastest, easiest and least harmful way to seperate the rusty top from a solid lower panel is to grind away the spotweld using a thick zip disc. It takes practice but once you know what you are doing it really works the best. Do as many as you can, and then slide a thin chisel type tool between the two panels to pop them apart.
You can usually see how far to go because you will see a halo when the weld is gone. Done right, the old top panel should pop right off and the lower panel will be unharmed and smooth.

I have used these too.They look like a cut off wheel but much thicker,1/4 in thick.I buy them at a paint and body supply store and mount it on a die grinder.A lot of sparks but work well.