Raise the car onto car blocks, and ensure the area has adequate ventilation before beginning work. Remove excess grease and rust from the underside of the vehicle. Prime and paint rust spots and allow them to dry, then liberally apply undercoating with a paint brush.
To prevent slipping, place car blocks securely under the chassis frame. Use a degreasing agent to clean the underside of the car, then grind and sand any rust spots. Clean and dry any rust spots before applying the primer. Apply undercoating to all areas of the underside of the car that are exposed to the road, let dry for at least an hour and then apply a second coat, if needed.