I have a radiator with a broken foot. Everyone I talked to about it said that it's impossible to weld cast iron. Not impossible as this tutorial shows, but not easy. I used my TIG welder for this, but you could also use a stick. In fact, I couldn't find cast iron TIG rods, so instead used a stick electrode with the flux removed.
Step 1: Grind Surfaces to Be Welded
First grind off the rust and paint with an angle grinder using a grinding wheel. Take a little bit of the edge off, about an eight of an inch so the filler has a place to go.
Step 2: Clamp and Tack
Clamp the pieces together. I used my TIG at 125 amps with a 1/8" cast iron electrode with the flux removed. If you can find nickel or cast iron TIG rods, use those. Cast iron is different than steel or aluminum. It bubbles real easy, and eats up the tungsten. After sparking the arc, back off on the pedal if you have one, otherwise use a lower amperage.
After tacking one side, remove the clamp and tack the other.
Step 3: Complete Weld
Be careful not to use to much heat and melt away your cast iron. Go slow and tap the filler rod into the puddle as you go. For this leg, I made four welds, on on each side. Afterwards, I lowered my amps to 60 and tried to smooth out my weld. This was where cast iron really differs from steel. It tends to bubble when steel would smooth out, but afterwards did in fact look smoother.
If you want it to look good, you'll need to grid it smooth as I did.
There you go! It is possible to weld cast iron.