How and what to paint metal for rust

Recommendation points

• What is the problem with rust
• When painting without stripping is possible
• Surface preparation
• Primer application
• Selection and application of paint

Most protective enamels require the preparation of metal surfaces prior to application, which is not always possible. In certain cases, it is allowed to use special types of paints that do not require rust removal. We will tell you about such paints and varnishes and the specifics of their use on metal for rust.

What is the problem with rust

Rusting of steel structures is a multi-stage process involving up to a dozen chemical reactions flowing into one another. There are three prerequisites for the appearance of rust: the presence of oxygen, water in any form and certain impurities in the steel melt. It is the presence of carbon or sulfur in the composition of steel that contributes to the destruction of the layer of the formed rust and the exposure of more and more new layers of metal. The process can be greatly accelerated in the presence of salts or with increased acidity. Sources of the latter can be solutions of carbonate and sulfurous acid in atmospheric moisture.

Unlike passivation, rust does not prevent further oxidation of the steel. The structure of rust is loose and hygroscopic, which contributes to the rapid accumulation of moisture and its retention. Thus, with the growth of a rusty layer, the course of oxidative processes accelerates: both due to a decrease in the conventional thickness of the metal, and due to an increase in the duration of contact with water and oxygen.

Usually painting metal for rust does not make sense. A certain amount of moisture and air accumulates inside the pores, which is enough for rusting to proceed even after processing. The density of rust is lower than that of steel, so high stresses are generated under the paint layer – the converted metal is continuously expanding. The coating quickly becomes covered with a network of through craquelures and flies around, enough damage to the paint layer in at least one place.

When painting without stripping is possible

The general complex of preparation of metal products for painting includes cleaning, grinding, coating with primer and a layer of protective enamel. However, cleaning is not always possible, in some cases it is easier to apply a coating with special characteristics. Let’s try to figure it out.

The first criterion that affects the ability to paint on rust, we considered above is the density of the rusty layer. More precisely, the ability of a product to accumulate corrosion activators. Over time, the variety and content of active chemical compounds increases, which is why, even in the absence of oxygen and moisture, reactions will still take place inside. It is impossible to predict how paintwork will behave in such operating conditions.

Another important factor is surface quality. A small uniform layer of rust, similar to a powder powder, allows the paint to penetrate to the metal surface and firmly adhere to the “body” of the product. But the presence of blisters and delamination on the rusty layer is almost guaranteed to lead to the destruction of the protective coating. Oil residues have a negative effect: if the rolled product has not been degreased, oil impurities can have an unpredictable effect on the protective layer.

Surface preparation

We smoothly approached the fact that rusty steel products require minimal processing before painting. How to prepare products – depends on their condition.

Surfaces ruined with small spots or even thin layers require dedusting and degreasing. It is enough to wipe them with a cloth abundantly moistened with solvent. It is also a good option to use aerosol degreasers: they contain corrosion inhibitors to reduce the chemical activity under the protective layer to zero..

Severely rusty surfaces need to be cleaned by hand: chipping off large detachments, scraping off with a scraper, then treating potholes and small shells with a metal brush. No need to polish to a shine: the surface may be covered with rust, but the last layer should be dense, durable and, if possible, even.

A prerequisite before painting for rust is thorough drying of the part. If possible, keep the product in room conditions at a humidity of 40–50% and a temperature of 20–25 ° С. If the product is permanently fixed in the open air – appoint painting after 3-4 days of exposure in warm sunny weather. The more moisture has time to evaporate from the rusty layer, the less the risk of premature damage to the coating will be.

Primer application

There are about a dozen ways to apply paint without removing rust. Most of this group of technologies involves factory methods of processing and preparing products, as a rule, we are talking about protective processing of heavy metal structures for construction. Privately, you can use two options:

1. Cover with an expensive two-component paint, having previously treated the surface with a passivating agent, usually containing phosphoric acid.
2. Opening with a universal primer suitable for application on rust, followed by painting with alkyd or polyurethane enamel.

The practical difference between these two methods lies in the cost of materials and the durability of the protective coating. Two-pack paints are applied in a thicker layer and form a durable film. Unlike conventional enamels, such a coating does not allow even minimal gas exchange; air and water vapor do not penetrate into rust. In addition, the paint has a higher hardness and at the same time remains elastic, it is able to withstand even very significant swelling of rust. In this case, the layer of corroded iron is constantly compacted, and the reaction subsides to nothing..

The budget type of coating in the form of primer and enamel ensures the preservation of the product for no more than 2-3 years and requires regular renewal. The latter consists in cleaning and degreasing the enamel layer, followed by applying the next one. The essence is the same: over time, the thickness of the paint layer becomes large enough to ensure complete insulation of the product from atmospheric influences.

Selection and application of paint

All paints and varnishes intended for untreated surfaces must have three qualities:

1. Weather resistance, formation of a film with a thickness of at least 150 microns.
2. Ability to inhibit corrosion (inhibition).
3. Ability to modify the composition of the existing rust layer for partial passivation.

As we have already said, the thickness of the coating depends on the type of paint, the resistance to various influences – on the quality and chemical purity of the base. Weatherproof paint can only be based on organic solvents. But with various kinds of additives and additives, the situation is individual..

The ideal result of the action of the protective coating is considered to be zero reagent yield during the deactivation of rust. All introduced substances must react in full, forming chemically neutral compounds as an output product. In reality, this is impossible, because the composition of rust is always different, it depends on the environment, and the grade of steel, and the conditions of its maintenance..

It is very difficult to develop optimal ratios of additives, and therefore their exact formulation is not disclosed. You can judge the quality by the complexity of the effects, the best indicators are:

• Passivators and top layer modifiers.
• Adhesion promoters acting not only at the rust-coating interface, but also in depth.
• Active substances that bind incoming oxygen.
• High-quality solvents that ensure deep penetration of paint and primer into the pores.

Ultimately, the result directly depends on the quality of paintwork materials. It is foolish to expect that cheap enamel paints such as PF-266 will provide high-quality and reliable long-term protection. In order to protect against the development of existing corrosion, only weather-resistant and special paints are suitable, in the marking of which, after the letter code, the number 1 or 5 is indicated with a hyphen, respectively. Alkyd and polyurethane enamel paints are the most suitable type of base to resist corrosion..

As for the method of application, most enamel paints are universal in this regard. The label invariably contains pictograms that determine the permissibility of working with a roller, brush and spray, so the final choice must be made based on the size and shape of the processed products.