I have 2 secondhand radiators, one with a few holes in the core but good end tanks, the other had a good core, but the drives side end tank has signs of being too hot and the inlet had a crack in it.
So I decided I would take my chances and pull the endtanks off and make 1 good radiator.
This is what I found.

The cores were over 50% blocked and there was a heap of powdered gunk in the end tank.
This is the cracked inlet on the good core (externally)

This is the damaged core that had the good end tanks. It has obviously had good coolant throughout its life.


So I hooked up the cold end to a hose and with water flowing, I used a hacksaw blade to clean the cores on the hot end (usually the only end that blocks up)
Once I had finished, there was clean, even water flow from every core, but I didn't take any more photos cos it got too dark
Now I just have to re-attach the end tank to the core