How To Fix A Rusty Radiator?

First drain it and start the motor with the water hose running in the radiator the whole time until the water is clear then add some of the acid designed to do that from an auto parts store and after putting it and water in till it is full let it idle with the cap back on the radiator for and hour or what it says!

Then repeat step #one.

Then add the proper coolant recommended by the maker, and put the cap back on.

Check the water level a lot for a few days for sometimes that rust was stopping a leak!

Logic101

If you believe the radiator is still in good enough condition(to save) I would have it flushed and cleaned and repainted after being checked for any defects under  water  with air pressure test(this is done with the radiator removed from the vehicle)...now if you already have the radiator out of the vehicle and find it is damaged beyond repair you can just put in a new one..this use to be a really cheap way to fix a radiator(having it flushed,checked and repainted)...now I do not know what this testing/repair  costs....you could also have the radiator re-cored but I would check the price difference between repairing and replacing with a new one...get at least three different quotes/prices....