How to choose and apply a primer for metal on rust?

Corrosion-resistant primer, which is applied over rust, is a modifier primer. It is used for the primary treatment of ferrous metal that has undergone oxidation. After application, a rough, uniform surface is formed, which is coated with insulating primers.

Operating principle

A feature of modifier primers is the presence of rust converters in their composition. Most often it is orthophosphoric acid. The principle of operation is based on the fact that the products of oxidation (rust) enter into a chemical reaction with the components of the primer and are converted into non-oxidizable compounds. That is why anti-corrosion primers can be applied over rust without cleaning the metal to a shine.

The converted oxides form an additional protective film on the surface with good adhesive properties, so that the subsequent protective coatings adhere well to the treated metal. The above properties are especially important if you need to process surfaces in hard to reach places where it is problematic to completely remove rust.

All rust primers contain surfactants that increase the fluidity of the composition and improve its penetration by a thickness of up to 100 microns. However, loose, crumbling rust is not able to convert any chemical substance, so it must be cleaned, leaving only a hard-to-remove layer of oxides.

Where can a modifier primer be used?

Dross is resistant to phosphoric acid, therefore it is not recommended to apply a primer containing a rust converter to such surfaces. If black rust forms on the surface, this indicates that the alloy contains magnetite, which is also resistant to modifiers.

You can not use compounds with rust modifiers on surfaces where oxidation has occurred partially and most of it is metal without corrosion. Non-peeled paint, like scale, will also not interact with the converter, and the primer will not be able to penetrate it. If during operation the metal structure was affected by sulfur, hydrogen sulfide or ammonia compounds, then the use of orthophosphoric acid primer is also impractical.

Types of primers that can be applied to rust

At the moment, on sale you can find several types of anti-corrosion primers.

• Primary, converting oxides and forming a thin film.
• 2 in 1 - performs two functions - primary primer and filler, creating a substrate with good adhesion under the paintwork (paints and varnishes).
• 3 in 1 - primary and secondary primers plus topcoat (enamel).

Enamels 3 in 1 - a good option if the work needs to be done quickly. When using them, there is no need to select a secondary primer and coatings. It is enough to apply 3 layers of such enamel, and it will perform several functions. Using a 3-in-1 primer-enamel, it should be taken into account that the absence of step-by-step processing reduces the period of anticorrosion protection of the metal to 5 years.
b

Tip

Combined processing with the application of primary primer and filler primer with the observance of the technology will protect the treated surface for up to 20 years, so it is better to give preference to this method of combating corrosion.

Rust primers can be one-component and two-component.Two-component should be mixed immediately before use, introducing a hardener to the main composition.

Surface preparation

All types of rust primers have general recommendations for surface preparation. It consists of several stages.

1. Mechanical cleaning of metal from the old coat of paintwork, scale and loose rust. At home, use steel brushes or a grinder for this, in industrial - sandblasting machines. In hard to reach places, abrasive compounds can be used, followed by washing with water under a pressure of 6-8 atm.
2. Sometimes for cleaning old coatings and scale more suitable option is heat treatment. In this case, an oxygen-acetylene burner is used. This type of surface preparation is chosen if there is a need to remove mill scale. In this case, cleaning with fire is more effective than mechanical cleaning.
3. After cleaning from loose rust and old paint, the surface must be degreased. This can be done with any alkaline formulations or special washes (solvents), depending on the type of stains on the surface.
4. The final stage of preparation is the distillation of water residues and drying. To remove moisture, use compressed air or scrapers for driving, drying is carried out either in special chambers, or using a building hair dryer.

Application technology

Features of the application depend on the type of primer and are indicated by the manufacturers. Two-component are mixed in the proportion specified in the instructions, and, if necessary, diluted to a working state with a diluent.

For both single-component and two-component soils, the diluent is selected based on the binder:

• epoxy - ethyl acetate or xylene;
• pentaphthalic - solvent or "White Spirit";
• polyvinyl acetate - condensate (demineralized water);
• polymer (styrene) - solvent P 4, P 646.

In single-component formulations, up to 20% diluent is added. Two-component are brought to the desired viscosity after mixing with the hardener, the amount of diluent ranges from 10 to 20%. After the desired viscosity is obtained, the primer is filtered through a funnel with a strainer.

The prepared primer can be applied to the prepared surface with a brush, roller or spray. Small ferrous metal products can be dipped. The peculiarity of applying primary primers is that they are rarer, therefore a nozzle of up to 1.5 mm is used in the sprayer. The application layer should not exceed 15 microns. Visually, it should shine through a bit, and not completely overlap the metal. Secondary soil can be applied no earlier than 20 minutes - this is the minimum time required for the reaction (conversion) and film formation.

For primers 2 in 1 or 3 in 1, a nozzle of 1.5-2 mm is selected. For working viscosity, a diluent is also added to them, but not more than 5% of the enamel volume. They are applied as well as primary soils, but the thickness of one layer is 40 microns. Processing is carried out in two to three runs with interlayer drying at 1 hour. Full drying time - 2-4 hours, depending on humidity and air temperature.

What is the primer consumption?

The consumption of the modifier primer largely depends on the condition of the surface being treated. To calculate the required volume, you can focus on such indicators:

• primary primer, except for compositions with an epoxy base, 110–340 ml / m² on each layer;
• epoxy - 120-150 g / m²;
• 2 in 1 or 3 in 1 - from 80 to 150 ml / m² on each layer.

Which primer to choose?

Choosing an anti-corrosion primer, you need to decide on the following indicators.

• Where will the soil be used - for external or internal work? For internal work, quick-drying compositions are selected that do not contain toxic substances.
• The aggressiveness of the environment in which the workpiece will be operated. For aggressive environments it is better to choose an insulating type soil with good hiding power and forming a dense film.
• Desired life without additional treatments. If you want a steel product to last for many years, it is better to stay on the primary primer. If the main goal is to reduce labor costs, you can purchase a 3-in-1 primer-enamel.