Why Important
Iron oxidises rapidly in the presence of water and oxygen and requires protection against corrosion by painting, As long as a paint film is unbroken, the protection will be effective.
What to look for
Rusting will usually begin in crevices and in joints where it is difficult to paint.
Although cast iron is the ferrous metal least susceptible to rusting, corrosion is exacerbated by poor maintenance and unsuitable design.
More elaborate precautions against rusting may be advisable if the ironwork is unusually intricate or of particular architectural or historic importance.
When to look
How to fix
- Restoring Ironwork – to be added
Restoring Ironwork
Method A: Dismantling and Cleaning
A recommended method requires the following treatment after dismantling and cleaning-off of old paint and rust:
All surfaces that are to be fitted together are pre-coated with a zinc-rich composition. Following re-assembly, the work is lightly grit-blasted, and flame metallised with zinc. This gives excellent corrosion protection without obscuring the surface detail of the work, even including subtle hammer-marks on wrought iron. This flame coating is followed quickly by a sealer primer, usually based on zinc chromate or zinc phosphate.
Painting can then be carried out in the normal way. External Painting
Method B: Restoring in situ
Another method is to use a suitable tannic acid- based rust inhibitor prior to painting. These are normally inexpensive and available at car accessory shops.
The surface of the ironwork does not have to be dry or competely free of rust, although all loose, flaking paint and rust should be removed, using a stiff wire brush. The rust inhibitor is applied undiluted in a single coat with a brush. It should be left for twenty-four hours or until the surface has turned to a dark blue-black colour. The ironwork should then be rinsed over with plenty of fresh water and allowed to dry before painting in the usual way.
This method (which was used by the National Museum of Antiquities for Scotland for the conservation of Mons Meg) is recommended for use on decorative ironwork in the Edinburgh, Scotland, and is particularly cost-effective, since the life of the paintwork system is considerably extended.
The main principles of rust removal and prevention are set out in External Paintwork
Avoiding creating problems
The Planning Department recommends that railings be painted black. Listed Building Consent is required for changing to any other colour.