Inspect the plate to verify it only needs to be refinished and not repaired. To repair cracks or fissures, the plate needs to be removed and the job handled by a professional piano repair shop.
Remove the old finish using the steel wool. Steinway plates carry the company's logo and are finished in bronze. Cover the strings and surrounding area to avoid affecting them. Gently rub the plate with steel wool, starting with a coarse grade. Rub in circles and do not use sandpaper which can scratch surfaces in a way steel wool will not. As the finish comes off, vacuum it up. Move to finer grades of steel wool as needed.
Remove any rust from the plate using naval jelly. Follow the instructions on the container and don't get any jelly on the piano's strings. Gently remove the naval jelly when the rust has been removed, after about 15 minutes.
Refinish the plate using the new finish. The original plate was finished in bronze, and because it holds the strings in place, affects the piano's sound. It is therefore best to continue the high-quality finish by purchasing a new finish from a Steinway piano supplier.