Warnex. I get it applied for me. Environmentally friendly, creates many different textures depending on application, available in a wide variety of colors, and easy to patch up down the road. Also, unlike some of the other "durable" coatings, it doesn't tear up my knuckles when I'm moving boxes around.
I don't know about using all this bondo to fill in holes/cracks/voids...
Seems kind of janky to me. If I was going to repair anything and wanted it to last, I would do some sort of fiberglass with cat hair or some type of high quality filler that is designed to last. Bondo is just about the cheapest and red-neckiest way to fill in dents and holes in cars, not to mention it sort of just falls off and cracks after a few years.
Also, "herculiner" is terrible.. it pretty much just flakes off if exposed to sunlight. Sort of makes sense that JBL uses an epoxy type covering instead of a rubberized urethane.
It will fill very small voids, but it's still best to fill them all. Seems like every time I think "Duratex will fill that" it doesn't. Not much fun to fix it after you've applied the top coat. It does not sand nor scrape well. Easy to touch up though.
What about grills? All the boxes I'm redoing (Versarray) have a shiney black and silver speckled finish on the grills. It seems pretty durable, except for the fact that I don't really care for the silver, plus a few have gotton scuffed up and look weird from a distance. I've held not doing them because I'm not sure I can get as tough a finish from a spray can of Tremclad.
Duratex does NOT work over standard wood putty either. It will crack and shrink back over areas of wood putty. The guys in FL actually recommended drywall mud as a filler.