Regarding Matteo's request for info on clearcoat removers, (and Kim's efforts to gently strip his mystery frame so as to get a look at the decals that remain), yes they exist. Also there have long been special varnish removers and decal removers designed to affect the offending material leaving the substrate undamaged.
However, as with and most anything containing solvents or VOCs, local regulations will determine if you can get them in your area. If you can't get useful information from your local autobody supplier and/or woodworking-furniture restoration supplier, you might contact the local art museum. They often have a small crew of folks expert at removing just one layer of finish and can be very generous with advise and information.
Gene Powell
Rad Finishes
Portland, OR