How to smooth the surface of parts printed with Co-polyester (PET) filament
I've found a chart which covers several plastics and solvents and only two of them (Chloromethane and Chloroform) are rated "D" which includes dissolving the material and both seem to be quite nasty and I doubt you will be able to purchase them without being placed on several lists.

Is it possible that something like XTC-3D from Smooth-On would work for you?

Also some more information on dissolving PET here, several sources also mention PET is affected by Hydrogen Peroxide but they do not mention to what degree the plastic is affected.