Fertan is the German-developed rust converter, manufactured in Germany over 30 years ago and used across the European classic-car restoration trade ever since, that has built a strong reputation for its ability to chemically stop and convert existing rust in situations where complete metal renewal is not practical. Rust is removed by chemical conversion: the active iron oxide is transformed into a stable, inert compound that no longer attacks the surrounding steel. The product is water-based, a tannic-acid and surfactant solution, and is applied directly to rusty steel, where it reacts with the iron oxide to produce a stable black iron-tannate film. The converted surface is no longer actively rusting and can be painted over directly without further treatment.
How Fertan Works
Conventional iron oxide (red rust) is unstable in the presence of moisture, it continues to react with the underlying metal as long as moisture is available, progressively consuming the steel. Tannic acid reacts chemically with the iron oxide, forming iron tannate, a stable, water-resistant, black compound that does not continue to react with the metal underneath. Fertan delivers the tannic acid to the rusty surface in a controlled formulation that also includes a wetting surfactant (helping the solution penetrate into pitting and crevices), a chelating agent (binding free iron in the rust), and a slow-acting catalyst that drives the conversion reaction to completion over the 24 to 48 hours after application.
Application and Use
Fertan is applied by brush, by spray, or by injection into closed cavities. The surface is prepared by wire-brushing or sanding to remove loose flake rust, heavy scale is not the right target for the product. The treated surface is left to cure for 24 to 48 hours at workshop temperature, after which any residual product is rinsed off with clean water (this is a water-based product, not solvent-based), and the surface is allowed to dry fully before painting. The dried, converted surface can be painted with any standard primer-and-topcoat system, Fertan does not interfere with subsequent paint adhesion, unlike some older rust-converter products that left a non-paintable film.
Specific Use Cases on a Classic
Fertan is particularly valuable in three classic-car restoration situations. First, on the inside faces of fitted panels, where a workshop cannot strip back to bare metal because access is limited but where the existing rust needs to be stopped before the panel is re-protected. Second, on chassis rails and box sections behind suspension components, where removing the suspension to access the steel would be a substantial undertaking but where injection of Fertan through a small access hole reaches the surface. Third, on heavily-pitted but structurally-sound steel, where a wire-brush preparation has removed the loose flake but the pitting holds residual rust that conventional paint would simply seal in rather than resolve.
In each case, Fertan converts what is there into a stable film that can then be re-protected with appropriate paint or cavity wax.