I have not been very successful with irons or guns on brass models. I have been most successful with brass repairs with electric resistance soldering. This concentrates the heat at the joint. I have learned that both surfaces need to be clean before the attempt.
Your plan to clean well and tin sounds very good. I am not real experienced with this so my lack of success with irons is probably not a good qualifier.
You do need to be careful not overheat the surrouinding surfaces and distort the brass panels. so the hard part is getting enough heat to melt/join the solder without getting so much as to damage the parts. And both parts have to get hot enough to melt the solder.
