Have you ever heard, "it's not how much solder you use, it's how you use it?"
Though in this day and age, I suspect most readers may be more familiar if I said, "it's not how much silicone you use, it's how you use it."
Copper brass radiators are constructed using solder, therefore it is very important the joints are designed and prepared correctly. That incudes meticulous cleaning and hand "tinning" with a tinning compound.
I import my tinning compound from Damon Industries in Ohio, the makers of RedKote Tank Liner. The reason I use this rather than the locally made brand is two-fold. Firstly, it is lead-free with less fumes, which makes it safer to use. Secondly, it flows better and covers much more effectively.
Once the joints are tinned, it is just a matter of running a first layer of solder (which actually all but disappears under the tank to build against the inside of the tank). A second layer is then the build-up we see after the job is done.
I created this section after stripping a radiator from an early V8 Ford Fairlaine, you can see from the pictures the build-up of solder inside the lower tank!
Sometimes less can be best.