Hold up - you mentioned using a split jig made from scrap plywood, but what kind of blade were you running on that 15 year old DeWalt? I've seen guys try to push a dull 60 tooth blade through thick crown and it puts way more stress on the jig than it should (since the blade gets hot and warps, the wood binds easier). Did you check if the arbor nut was torqued down right before you started cutting? That little hairline crack might have been there from a previous cut where the blade wobbled just enough to start the damage.

Actually, about the epoxy primer thing - you gotta be careful there. Modern epoxies are miles better than the old stuff, sure, but they're not a magic moisture barrier if you've got thin filler over bare metal. I've seen guys slap on a skim coat of filler right over bare steel with epoxy on top, and it's fine for a while, then it starts bleeding through the paint a year later (little brown spots, you know the ones). The real trick is you still want to at least hit the bare metal with a self-etching primer or a proper metal prep before the filler, even if it's a super thin layer, just to kill any chemical reaction. Epoxy over filler that's on bare metal can trap microscopic moisture from the filler's curing process, and that's where the problems start in humid climates. So yeah, filler is totally fine for production work, but skip the metal prep step and you're gambling on that $200 holding up long term.