Although it is a very labor intensive task to make such a radiator almost by hand , it is also very interesting to see it comming together.
I am sure there are other ways to get the job done more quick, but here is how I continue for now.

Very first radiator main housing frame put together

Each and every single radiator core tube is cleaned using a brass wire brush.

Each and every single radiator core tube is tin plated by hand in a tin bath to ensure the soldering process will result in a leak free radiator core. In addition this will help to safe some weight by not adding too much extra tin during soldering.

Stages in thelife of a radiator core tube.
Enjoy!
Achim