I'd be very worried about warping of a thick ply face on a fire door, even
with a glued and screwed construction. I put in a new claimed hardwood,
but
actually balsa wood front door 4 days ago and it has already warped by
about
3mm.( I lied, 5mm) I think that the suggestion to surface bead the face of
the existing door might be safest. Most fire doors I believe are internal
use specified and may not like humidity changes. If you decide to go
ahead,
I would screw through the whole door from the inside with some "pretty"
screws, or, lose the heads in brown screw covers. It requires careful
selection of the screw length to not penetrate the front of the ply and an
accurate pilot hole will be required. Feasible, but not easy. I don't
think
I would glue the ply in place, a bit of movement might be a good thing.
It's
easy to add the glue afterwards if it seems necessary.
Regards
Capitol

I do plan to seal/varnish heavily - do you think this will provide a level
of protection from warping, etc?

I've got another blank as an outside door - it's been on for a couple of
months now. It did "move" a little over the first week or two and i needed
to adjust the strike plate, but it's settled now! I had ronsealed the
outside face, but not the inside.

I thought about screwing from the inside and carefully countersinking and
plugging the holes with plugs made from offcuts of mahogany.


thanks
paul