Tony, all I did with the brass right angle edge joints of the tender for the TE (needed caulking to make them watertight for the tank), and the beading at the top, was to prop then up. As you said I used a soft flame and kept it just ahead of the solder, so it ran down the joint under gravity. It didn’t run anywhere, . I did paint the joint with Bakers fluid before heating, and it was terrilby easy. I did get along the beading a couple of places where a slightly thicker “blobette” built up, but a scraper took that off without trouble.
The solder – oh some old cored stuff I picked up a derelict workshop about 8 years ago.
If you have to fill a joint, proper plumbers solder might be better, because with the judicious application of heat, you can keep it at the pasty stage very easily. Its very controllable , so you can literally get an effect like gas welding, where just the little pool is molten and you can work your way along bridging a reasonable gap. Try it with a bit of scrap and I’m sure with proper LEAD plumbers solder you’ll get the hang in a few seconds.
You can do the much same with silver solder too if you wanted to, being a eutectic, though EF is not the best alloy. Anything with a long melting range, but EF will work at a pinch.. Probalby do a far stronger job, and much less prone ot crack in service.