The burnishing fluid I got is this:
It appears to be a solution of some type of acid that interacts with the finish on the brass, turning it black. I've seen online that it's the same as the chemical used for gun bluing (although not living in America I'm not sure how easy that is for you to get either)
I agree with Brian - the finish is nice, but it's quite delicate. When I first handled it I saw that it began flaking off. I had planned on using gunmetal pigments again like I did on the plastic parts of the guns, but seeing how fragile it was I decided to try lightly buffing it with a brush, and that worked well so I left it alone. Yeah, the problem with these tiny PE parts and brass barrels is that they're really hard to sand. I used self- etching primer on the kit PE, which seems to have done a nice job of priming those parts - haven't had any difficulty with paint adhesion. But for these I thought I'd try the burnishing solution instead.
This bomb bay has really been a bugger for me. First I've spent an inordinate amount of time just looking at it, and trying to decide how I was going to proceed with assembling, masking, and painting. I initally decided to leave the doors and actuator arms off and just tape over the bay itself. So I installed the sidewalls without following the kit instructions to put the arms on first. Now I've decided that the open doors can actually just be built into the masking, so I'm working to install them. I got the idea of leaving the doors off til later from Brett Green's book. What he did not mention is that, while possible, it's quite tricky to get those arms onto the sidewalls in the right alignment after they're already on the kit. I've got them on, but I'm still not sure if they're right, or symmetrical from side to side. So, we'll see.
Finally, another question: the kit instructions call for the inner surfaces of the engine cowling panels to be painted AS-12, or bare metal silver. Since there were some other inaccurate color callouts on the gear bay, I was wondering if this is accurate, or interior green? I can't seem to find any pictures of the inside of the cowling panels on a real plane.