Whether rebuilding an engine or just restoring a serviceable engine, rust is sometimes present and needs to be removed. There are many ways to remove rust from cast iron. Normally a little elbow grease and a chemical rust remover can do the job. However, for an engine that is going to be rebuilt, let the machine shop handle the chore.
Disassemble the engine to the bare block. Choose a machine shop that you trust to do the work. The machine shop will vat and shot-blast the block. This removes all grease and the rust. The block can then be painted.
Treat rust areas with Naval jelly if you choose to do the job yourself. Apply Naval jelly with a brush. Allow the chemical to sit for 30 minutes. Scrub the area with a wire brush if the rust is heavy. If the rust is still present, repeat procedure several times until the rust is removed.
Apply Evapo-Rust to the rusted areas as another option. Soak a rag or the sponge with Evapo-Rust and let sit. The amount of soaking time corresponds to the severity of the rust. Scrub with the wire brush between soakings for stubborn spots. If possible, degrease and clean the area before applying Evapo-Rust.
Purchase a small sand blaster to remove the rust. This may be more economical than using the machine shop if you have several blocks to do. Obey all operator safety precautions as prescribed by the sand blaster manufacturer.
Clean the areas with a solvent once the rust has been removed, and apply primer or paint to prevent rust from reappearing. Rust is preventable and all metal objects should be checked periodically for rust.
Obey manufacturers instructions when using chemicals. Wear safety glasses and gloves.