Sand Man your gun looks great! Birchwood Casey makes an inexpensive cleaner you can buy to prep before painting...I believe it is actually prep step before using cold blue to degrease.
I tried automotive lacquer thinner once and I think it is too strong...it actually dulled the blue finish....fortunately it was only on the magazine tube and hidden under the forend. It woudld not recommend that as a cleaner or prep on a gun.