To repair a truck frame, cut the rusted portions of the frame, make a template of the cut pieces, and use them to cut the replacement metal. Make the pieces rustproof, and weld them on the appropriate places on the frame.
Before cutting the rusted parts of the truck's frame, determine how rusted it is, inspecting the entire frame. Remove any welded pieces using a cutting torch or grinder. Then remove old paint and rust as much as possible by sanding manually or using a sandblaster. Look for any cracks or blisters on the exposed metal.
Cut the rusted portions of the frame with a hacksaw, grinder or torch. To create a template of the cut pieces, place the metal pieces on cardboard, and cut out the shapes with a sharp knife. Place the cardboard cutouts on a metal piece, and spray paint all around them. Take out the cardboard pieces, and cut carefully along the edges of the shapes created. Sand to smooth the edges of the metal patches
Using a rustproof product, follow the manufacturer's directions to make the cut metal patches rust-resistant. Weld the metal patches strongly on the appropriate places on the truck's frame. Sand and grind the weld to smooth it. Apply a coat of primer to the frame, and allow it to dry. Follow this with a coat of fresh paint.