Loads of old panelled Victorian doors in our house. OF course, these
have been painted many times and the panels are immovable and often
cracked. Filling and painting is a bit Forth bridge like.

If I had some thin material which wouldn't crack and looked like wood or
maybe was wood, I could cover the panels with this. The slight loss of
depth probably wouldn't show.

What might work?

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Cheers
Clive