Looks like I'l actually have to go through the pain of contacting either Trenton or JC. No point in buying what might be wrong. Those mineral core luan doors come in ratings from 20 to 90 minutes. Hard to believe a steel door with a lumber edge has a poorer rating than a wooden door with a varnish. Especially the panel door I have, I doubt it was meant for that. It's not even particularly secure!

I put in metal framed, metal 6 panel doors that have a 1 hour rating listed on them. This met code way back when I renovated (15+ years ago).

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I believe you answered your own question. The "varnish" you are talking about is probably an intumescent coating, which as I recall was used to fire rate wooden doors with flat panels and 1-3/8 thick. The Fire Department accepted this as a 20 minute rating, in cases where the frame could not be modified to accept a 1-3/4 door. They may or may not accept this option, so you really should call them. What's confusing here is the term "mineral core" door you are referring to. That fill is generally used in a 90 minute or higher rating, and further requires a steel frame to maintain the fire rating. You should really check with the Fire Dept on the required rating, as it is dependent on many factors. Size of building, number or units to name a few, and they should be able to tell you right off, age of building may or may not have an impact.

Never had to replace a door before, I got a mineral core door warped beyond redemption. Has anyone had to deal with what the rating requirement is for old buildings, not new construction? Last time I had to deal with this issue I painted a wooden door with fire rating varnish, and it passed green card.