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The rusting of metal is a major economic problem. It has been estimated that as much as one fourth of the steel produced in the United States goes to replace steel products and structures that have been destroyed by rusting and over 2 billion dollars is spent each year in the USA on coatings that are used for rust prevention.
Rust is the result of a conversion process that takes place when oxygen and moisture come in contact with metal. The oxygen and water combine with iron to create iron oxide the familiar brown coating on the exposed surfaces of the metal commonly known as rust. Left unchecked, moisture and oxygen (in the atmosphere) will continue to pass through the outer layers of iron oxide to attack the unrusted steel beneath. So over time, a piece of steel just keeps rusting and rusting until it has completely rusted through and crumbles under the merest touch. This chemical process can be stopped if water and moisture are not allowed to contact the metal surface. Before we get into the various refinishing techniques lets go over basic surface preparation which applies to all of the finish systems.
Regardless of which method of rust prevention you choose, proper preparation is the key to success. Approximately 80% of all paint failures is due to improper surface preparation so this step should not be overlooked unless you want to keep doing this over and over.
Rust Removal:
The surface must be cleaned of all loose and flaking rust and paint.
Sandblasting:
Sandblasting is the easiest, quickest way to remove old paint and rust from metal. Compressed air at high pressure is used to blow fine sand or other abrasive material through a hardened spray nozzle and quickly "blasts" away whatever the blast material hits. Sandblasting is ideal for deeply carved wood, metal or brick surfaces, ornamental iron work and hard to reach areas. Small handheld units are available for small jobs like tools, wrought iron handrails, outdoor furniture etc., a compressor is required which you can rent from a local tool rental supply company.
Wire Brushing, Sanding:
Use a hand brush or a wire wheel brush and drill. Normally a few quick passes will remove the loose material. If the surface is to be finished in a smooth painted finish then the wire brushing should be followed up with a complete sanding until it is smooth and free of defects. Normally a rough sanding with 80 grit sandpaper followed by a smooth sanding with 120 grit does an adequate job. Holes, cracks etc should be filled first with an auto body filler material if you are going for a top quality finish. Wrought iron handrails, lawn and garden equipment etc normally do not require a really fine finish whereas a bicycle, patio table, metal door and the like would.
Chemical Removal:
Acid or strong alkaline materials which dissolve rust. These products work well but must be used with extreme caution due to the chemical nature of the products, they are ideal for large areas like metal roofs and buildings.
Rust Conversion:
Organic Rust Converters:These products convert rust to harmless chemical compounds and deposit on the surface a protective film that protects against rust.
Pros...No serious wire brushing or sanding is required, these products actually need a rusty surface in order for them to work properly. Water Base, Easy to apply, non-flammable. Rust Converters are ideal for ornamental iron and other areas where wire brushing and sanding is difficult or the surface is not easily accessible. They not only convert existing rust to an inert organic surface but also deposit a rust inhibiting polymer on the surface which does not require finish paints or coatings, this is the ideal treatment for ease of use with minimum surface prep required.
Cons...The downside to rust converters is they are thicker than normal coatings and tend to leave a high build, rather "ropey" looking finish, ( looks like paint would with a lot of brush marks). This would not be a problem on wrought iron or any other surface where a super smooth finish is desired.
Cleaning:
Next the surface must be cleaned and degreased, paints and coatings do not bond well to dirty or greasy surfaces. A washing with a strong detergent followed by a through rinsing is required. The light "flash rust" that appears after washing can be removed with a cloth dampened with paint thinner or one of the commercial "surface prep" materials available.
At this point you can coat the metal with a simple oil or grease like WD-40, a good application for something like gardening or hand tools.
Painted items should be primed with a rust inhibiting primer and finished with two coats of a quality exterior enamel. Spraying is quite acceptable but I strongly recommend that the primer be brushed or sprayed and worked into the surface with a brush while still wet. Spray painting alone will not get the paint down into tiny pinholes and crevices but will only "bridge" these areas resulting in premature failure as the moisture and oxygen will attack the exposed steel.
Modern paint chemistry now allows water base paints to be produced which have fantastic rust inhibiting properties and I highly recommend them over solvent base. Besides being low odor and easy to cleanup, an added advantage to water base paints is their ability to melt in with any trapped moisture which may not be visible to the naked eye. These coatings will force the moisture up to the surface and replace it with rust proof chemicals that seal the surface off. You will see rust looking spots in your dried primer but do not be concerned, most can be wiped off with a cloth as it is nothing more than rusty moisture that has been forced to the surface.
Summary:
Regardless of which finish system you use, preparation is a must!
For painted finishes, prep, apply rust converter, or metal primer and two coats of rust inhibiting paint such as POR-15 Rust Paint.
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