Rust Converters

A rust converter is a liquid treatment applied to rusted steel that chemically reacts with iron oxides to form a more stable, darker compound that can be overcoated. Most products are based on tannic acid, which forms iron tannate, or on phosphoric acid, which forms iron phosphate. Many combine the active chemical with a polymer, often an acrylic or vinyl latex, that forms a thin primer-like film. Rust converters are used for maintenance, vehicle restoration, agricultural equipment and small repairs where abrasive blasting is impractical.

How rust converters work

Tannic acid products

Tannins are plant-derived polyphenols. They react with iron oxides to form iron tannate, a blue-black compound that is more stable than rust. The reaction darkens the surface, which helps applicators see where the treatment has worked.

Phosphoric acid products

Phosphoric acid dissolves some rust and reacts with iron to form iron phosphate. These products are often called rust removers or metal prep when used to clean, and converters when left on the surface. They are related in effect to conversion coatings used on clean steel in factories.

Polymer binders

The polymer in many converters forms a thin, sealing film over the converted layer. This film improves adhesion of the topcoat and slows moisture penetration, but it is too thin to act as a long-term coating on its own.

When rust converters are appropriate

• Small areas of light to moderate rust.
• Inaccessible surfaces such as interior cavities, crevices and back sides of panels.
• Sites where blasting is prohibited by noise, dust, or fire restrictions.
• Vehicles, trailers and equipment in mild or moderate service.
• Temporary protection until full maintenance can be scheduled.

When they are not appropriate

• Immersion service, such as tank linings, potable water and wastewater.
• Severe corrosive environments defined in ISO 12944 as high categories (see ISO 12944 corrosivity categories).
• Heavy, layered or flaking rust. Converters only react with a limited depth of oxide. Thick scale must be removed first.
• Mill scale. Intact mill scale is not rust and does not convert (see mill scale).
• Salt-contaminated steel. Converters do not remove chlorides and sulfates trapped in pits, which drive future corrosion (see soluble salt contamination).
• Projects specifying a blast cleaning standard. A converter does not produce SSPC-SP 10 or Sa 2½.

Application practice

1. Remove loose rust and scale with a wire brush, scraper or power tool cleaning. The converter needs a firm rust layer, not loose flakes.
2. Remove oil and grease with solvent cleaning or detergent.
3. Wash off salts with clean water if the steel has been exposed to marine or de-icing salt, then allow to dry.
4. Apply the converter by brush, roller or spray, following the label. Avoid applying to bright, clean metal unless the product allows it, since there is no rust to react with.
5. Allow the full reaction time. The surface usually darkens to black or blue-black.
6. Apply a second coat if the label calls for it.
7. Topcoat with a compatible primer or finish within the recommended window.

Temperature and humidity limits matter. Cold temperatures slow the reaction, and rain can wash off uncured product.

Topcoating

Converted surfaces should almost always be topcoated, because the converter film is thin and UV can degrade it. Points to check:

• Compatibility. Some acidic converters are incompatible with certain primers, especially some zinc-rich or alkaline products. Follow both manufacturers’ guidance.
• Residual acid. Excess phosphoric acid left on the surface can cause blistering or poor adhesion. Some products require rinsing.
• Film thickness. Build the topcoat system to the thickness needed for the environment.

Limitations and misconceptions

• “Converters stop rust permanently.” They stabilize surface rust, but corrosion can continue beneath if salts, moisture and oxygen reach the steel.
• “Converters are as good as blasting.” Blasted steel with a proper profile and a full coating system usually performs far better in demanding service.
• “Any rust can be converted.” Deep pitting, laminated rust and scale need mechanical removal first.

Comparison with surface-tolerant coatings

Surface-tolerant products such as epoxy mastic and some direct-to-metal (DTM) coatings are designed to wet and seal tightly adherent rust after hand or power tool cleaning. For maintenance painting of structures, they are often preferred over converters because they provide film build in the same step.

Key terms

• Tannic acid. A plant-derived acid that reacts with iron oxide to form iron tannate.
• Phosphoric acid. An acid used in some converters and metal preps that forms iron phosphate.
• Iron tannate. The dark, more stable compound that converted rust becomes.
• Rust-penetrating primer. A low-viscosity primer that soaks into rust and encapsulates it rather than converting it.
• Tightly adherent rust. Rust that cannot be lifted with a dull putty knife, the type converters are designed for.
• Neutralizing rinse. A water or manufacturer-specified rinse that removes residual acid before coating.

Frequently asked questions

Can I paint directly over a rust converter?

Usually yes, after the reaction is complete and within the recommended window. Check compatibility with the topcoat.

Does a rust converter need a primer?

Some products are designed to act as the primer; others recommend a separate primer. Follow the label.

Is a rust converter safe to use?

Converters contain acids and must be handled with gloves and eye protection. Read the safety data sheet.

Related entries

• Hand tool cleaning
• Power tool cleaning
• Epoxy mastic
• Corrosion science basics
• Direct-to-metal (DTM) coatings (new)
• Conversion coatings (new)