Selecting Cast Iron Stick Electrodes usually comes down to 3 things: price, machine-ability, and if the weld is solitary or multiple pass.
The professional supplier of China Cast Iron Stick Electrodes will tell you
Tech-Rod 55 (AWS class ENiFe-CI) is a nominally 55% Nickel electrode. The lower Nickel content makes this electrode more economical than Tech-Rod 99. Weld deposits are usually machine-able, but under conditions of high admixture, the welds can become hard and difficult to machine. It is often used for repairing castings with heavy or thick sections. As compared to Tech-Rod 99, welds made with 55 Ni are stronger and more ductile, and more tolerant of phosphorous in the casting. It also has a lower coefficient of expansion than 99Ni, resulting in fewer fusion line cracks.
Ferroweld (AWS course ESt) is a lesser price, steel electrode. The weld deposits are tough, and are't machine-able, but may be completed by grinding. This is the cheapest price electrode for welding cast iron, and the electrode has quite a user-friendly arc. It may withstand welding on castings that cannot be completely cleaned prior to welding. Ferroweld residue will rust, exactly like cast iron. This might be significant when fixing cast iron components like exhaust manifolds on classic automobiles.