unfortunately, yeah, most likely. Sure you have a good (IMHO) argument that things haven't changed significantly ... removing one polyurethane and replacing it with the same. This "same" detail is important; "similar" probably won't fly. Maybe check the AC, and what it has to say about repairs and refinishing ?

another day in paradise, or is paradise one day closer ?

So … MRP …
Why are you asking here? Did a DER or FAA person state that you need to run the test?

There is no data available. The interior was customized many years ago and there are no details about finishing in the paperwork. “Worst” scenario would be to build a surrogate panel as AC 20-178… and we are trying to avoid it.

If you don't have traceability for the finish that's on the panel already, then I'd say any "same-as" argument becomes impossible.

There is a FAA policy statement on the matter that is intended to help you in this situation. Sorry I can't remember exactly which it is. Try googling "FAA flammability test policy surrogate panels" and go from there. Also note that the FAA maintains a FAR 25 / Advisory / Policy cross reference web page. Find that page, scroll to 25.853, and you will be taken to many more links that will help you out.

An FAA policy statement we use constantly to show compliance to 25.853 is PS-ANM-25.853-01-R2 (the R2 is the revision level) as it has a number of FAA approved means of compliance. I am not an FAA DER/ODA UM for flam so I cannot speak with authority, though I write a LOT of flammability test plans that have to be approved by an FAA DER/ODA UM for flam.

Regarding the new finish meeting 25.853, the issue we typically run into is when a manufacturer specifies their material meets 25.853 they usually have their own internal testing but very rarely have "FAA-witnessed" flam testing. "FAA-witnessed" means an FAA DER/ODA UM for flam witnessed the accomplishment of an approved test plan with conformed coupons and then signed to find compliance for the test results. Even if they have FAA-witnessed testing on file, we then have to consult PS-ANM-25.853-01-R2 to see if the substrate the manufacturer used for test can be an acceptable means of compliance for whatever substrate will carry the finish on the aircraft. That determination belongs to the FAA DER/ODA UM for flam assigned to your program.

The issue as I understand it from my non-FAA-delegate perspective is that the finish and the substrate can interact in unpredictable ways for flam when combined, and thus the FAA is cautious concerning similarity statements. There are some finish/substrate combinations that suppress flam propagation and there are some finish/substrate combinations that accelerate flam propagation. Without testing, or alternatively showing an already accepted Method of Compliance for the combination via the PS-ANM-25.853-10-R2, it is an unknown that must be resolved.

Although the requirements are slightly different between 25.853 and 23.853, would the same case apply to a Part 23 normal certified airplane required to comply with 23.853(a)?

I think you should ask the DER who is making the finding.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.