I will go ahead and apologize for not taking photos as I went along making the exhaust. There were probably 50 steps to get them finished. I don't like the way Tamiya made the tunnel because it was very hard to paint being all one piece and closed at one end.
Here is my finished interior of the tunnel:
And here is the tool I used to make the grooves as in the reference:
I will try to explain what I did. First, to create all of the grooves I inserted a bolt through the tunnel to stick out the back and tightened a nut to secure it. I put the end of the bolt into the chuck of the drill and let it spin. Bracing my hand, I held the scribe and cut grooves every millimeter or so. Of course it became more difficult the further into the tunnel I went. After the grooves were done I drew 3 lines with a sharpie on one tunnel and four lines on the other tunnel so that I could line the up, then cut the ends off so that I could paint inside. See the lines in the next photo.
I painted using a brush with white, brown wash, steel, copper, and black wash, going in order probably four times, building up the look I wanted.
The turbos were painted by brush with steel, black, copper, aluminum, and steel with black added for a wash at the end. Same as before, I may have repeated the process 3 or 4 times.
The nozzles started out black, then I airbrushed Alclad Gold Titanium inside and out. The pieces just forward of the nozzles were treated the same. I added some Aluminum to the center band around the nozzle. I sprayed copper on the outlet side, and Steel on the forward edge. I thinned Gold Titanium with lacquer thinner and misted the entire nozzle. Then I did it all twice more. I taped off around the nozzle overlap section and sprayed that with Tamiya XF85 Black Rubber thinned 50/50 with Mr. Leveler. I removed the tape and with a small brush I painted the little ribs on the outlet with undiluted XF85. Inside I used Vallejo Model Color 70951 White to dry brush the raised ridges from both left and right. Then brushed XF85 between each raised ridge to get the white/black striped look.
I taped off the part of the nozzle attachment piece that would be fuselage color and primed with black, made random sqiggly lines with Alclad Highshine Aluminum #126 thinned with lacquer thinner using low pressure with the airbrush needle guard removed to make the tiniest of lines. Then I sprayed with Vallejo 61276 Model Air USAF Light Gray.
This assembly will go on after painting the aircraft.
I have decided to mask off the wheel wells and paint all fuselage color and weather before finishing the landing gear
I have to decide how to deal with the canopy and cockpit before painting the plane. Sorry for the long winded post!