Cleaning
Cleaning antique iron typically means removing built up rust. Gently scrub away the rust using a plastic or wire brush and mild solution. Possible solutions to consider include vinegar mixed with water in equal parts, paraffin oil and commercial derusting oils. Use emery cloth with oil as another alternative.
Protecting
After cleaning the iron, protect it to prevent future rust buildup or other damage. Seal the iron with a derusting fluid before painting it with dull black paint. For further protection, apply a lacquer coat.
Replacing Parts
If rust or previous use rendered a detachable portion of the cannon damaged beyond repair, consider having that part replaced. Doing so could negatively impact the cannon's value, however.
Professional Services
A number of professionals specialize in refurbishing antique metal. Services range from simple cleanings to advanced polishing to the recreation of missing parts. Research these professionals thoroughly before deciding whom to contact.