I’m afraid this might be an issue with mine. I’m on my second time flushing with garden hose before installing the new rad. I flush but it will sit a week (making more rust) while waiting for parts. I’ll flush again and hook it up. See where I’m at.
A guy gave me a receipe (homebrew with a caustic acid solution) then neurtalized. It worked for me on an old slant that really needed it. I was scared!!
Surface rust is one thjng, scale is another.
A heat gun aimed all over the radiator when hot will show the spots not doing their job. Gun at HF is cheap. I am cheap!