Home job Body undercoat?

Anyone ever done their own body undercoat for rust prevention? If so what brand and how tough is it?

Bondo brand has been the best to me.

I shopped around.
I like Bondo and 3M products a lot. Bondo's professional grade rattle cans are great but expensive.
I recently did the inner & outer fenders of another project with 3M's professional under coat, bought a gallon on Amazon for $65 and brushed it on. Not as chunky as the factory stuff but cures pretty hard, brushes on pretty thick and fills in lots of cracks gouges and seams pretty well. At $65 a gallon, your money goes a lot farther if you buy a paint brush & do it the old fashioned way.

Did a gallon do the whole underside?

This was on a '57 Chevy Half-Ton. A gallon did the front wheel wells and a bunch of stuff on the old Cherokee (I forget what) + the cab floor & back wall of the truck, some other junk, and I have a little left over.

Permatex makes a decent rubberized undercoating that is inexpensive. I used it after I got my frame stiffeners welded on. Duplicolor makes something similar for about the same price. Really depends on what you want to do.

I have 3m rubberized. It works well and looks good in the fender wells.