It looks like you might have an original fan. Have you considered replacing it with an aluminum replacement?
Is this a recent problem or has it been going on for a while?
Do you have overheating problems? How new is the car to you? Is this a new discoloration?
Clean the areas with a nylon detail brush or tooth brush. Stuff pieces of paper towel in the radiator at the suspected area and observe if the paper towel gets moist. If the paper does not get moist then paint the radiator with radiator paint. If it is moist then consider a radiator stop leak and once the leak stops, paint the radiator.
The stop leak needs the pressure to find and "push" the product into the hole(s). Best of luck to you.
Aluma-Seal works. Before there were fancy store-bought products, ground black pepper was the home remedy for radiator leaks.
Just be aware that the stop leak may or may not work. Most stop leak products are designed to work with a pressurized system. The stop leak needs the pressure to find and "push" the product into the hole(s).
Best of luck to you.
You are correct, -and a method we use is to take a 3" long piece of rubber hose and insert a small 5/16"bolt into one end of the hose, and slide the open end of the hose over the overflow drain tube. When the cap is reinstalled and the drain tube is plugged, it will generally only build 2-3 lbs of pressure inside of the radiator tank when the engine comes up to temps, but that is generally enough to force the Sodium Silicate thru the holes where it can find air to seal.
We also use this method on engines that are mounted on the run-in stand to seal threads in water jacketed areas during initial run-in.