start of body work

It's a rotary sander that is typically used in collision repair shops for cutting body filler quickly.

It's made by a company called Hutchins and it's not cheap. There may be other brands that people use the name as a reference to, but I know the name under that brand actually being used. I've also heard the term mudhog used for an air file, which is a reciprocating sanding tool that uses sanding board paper. I'm betting this is what he's referring to.

If you are doing this as a hobby, don't bother with an expensive sander. Just pick up a 6" DA (dual action/ orbital) from Harbor Freight, Tool King or the like. They are cheap, but they will work fine with a good pad. Don't use the skinny backing pad that they come with. Get something from a paint supply shop that suits the paper you buy.

I think epoxy primer is good for bare metal, but I don't like using primer on anything coarser than 80 grit on bare metal and 150 on filler. The scratches can show up later, unless you are willing to let the primer sit after sanding, to guide coat and re-sand.

I went through a roll of 80 grit on a 6" DA sander to knock down all of the paint to bare metal on the Scamp in a day, on my own.

Acid Etch primer is a good thing to put down, before using a urethane primer. Epoxy primer can go direct to freshly sanded metal.

I prefer polyester primer, because of it's high build and sand scratch coverage, coupled with its ability to give a uniform, chemical surface that bonds to metal and filler. It helps keep areas where filler is used from shrinking or moving at different temps that the car is subject to, on its own, because the entire surface is alike.

It makes a good barrier for a 2nd coat of urethane primer to go and helps get the car extremely straight, because you can block sand it with a 2' long board, over gaps, with anything from 150-320 grit, before a 2nd coat of primer, for final fine sanding.

It also replaces the need for polyester putty/ glazing putty on filler areas, as long as you are vigilant on removing air bubbles from the filler, when mixing it, by spreading it from the top on the mixing board. You can go back in and correct pinholes in the filler, if you find any, by taking some of the polyester primer with your glove and dabbing it over the pinhole a few times. It sands out just fine.

If I were going to prep a car for a complete paint job, I wouldn't hesitate taking it all down to bare metal, unless I had no place indoor to store it. You will end up with a nicer finish in the long run. The more uniform the surface is on a molecular level, the less likely you are to see areas changing independently after time.