Paul J S Green wrote in message ...etc

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