How to remove paint without damaging the original one

I bought this Trek 800 1991 at a LBS and the owner painted it black because it was all scratched. I was removing the fake decals and the black paint came off revealing the original white paint. I am removing it with a pocket knife, but it's hard when the black paint is not loose. Any way to remove the black one easier without damaging too much the white behind? (I just want to see the how original paint is and maybe spray a clear coat to preserve the patina)