How to paint metal?
Preparation of metal structures for painting – the most important procedure on the quality of which depends on the quality of the future coating. The surface must not only be cleaned of dirt, but also in the final step degrease the metal before painting.
Preparing Metal for Painting Complete Guide
Stages of the work
Preparation of metal – not as simple a process as it may seem at first glance. The work is divided into several stages, the most important of which are:
- Removing rust and old paint from the surface;
- Performing phosphating and degreasing.
Preparation for painting metal products can be carried out according to different techniques, but first of all, rust and remnants of the previous paint layer should be removed from them.
Paint and Rust Removal
Cleaning corrosion and the old paint layer from metal can be done in three ways:
- Chemical
- Mechanical
- Thermal
Mechanical method
This method, which is considered the most effective, involves the removal of rust and paint by hand or with mechanized tools. The treatment can be carried out:
- Wire brushes
- Sanding discs
- By means of a sandblasting machine
- Water abrasion (only for industrial applications)
Chemical treatment
Chemical treatment is based on the action of the rust by spraying chemicals on the surface or by applying them with a brush.
There are two types of rust removers:
- Rust removers;
- Non-washable.
The disadvantage of rinsing agents is the likelihood of new rust spots on the metal, so after treatment, the surface must immediately be dried and treated with rust-preventive compositions.
When treating rust with non-washable compositions as a result of the chemical reaction the surface of the metal is a kind of primer layer, which can not be washed away with water.
Metal structures are most often treated with:
- a solution of sulfuric or hydrochloric acid (5%) with the addition of a corrosion inhibitor;
- phosphoric acid (15-30% essence), which converts the rust into a protective coating;
- a mixture of 50 grams of oxypropionic acid acid in 100 ml of vaseline oil, which turns the rust into salt and is easily removed from the surface with a rag.
Thermal method
Removing paint from metal surfaces with the thermal method uses a blowtorch. The metal is heated until the paint coating gradually peels off and is easily removed with a spatula or metal brush.
The main advantage of this method is a significant saving of time, while the main drawback is the fire hazard and some restrictions on the types of surfaces. Handle sheet metal, galvanized material, cast iron can not be by this method – the surface is deformed, violating the integrity of the structure.
Degreasing of metal
The degreasing of structures is carried out to ensure good adhesion of metal with the paint composition and primer.
In principle, any compositions that remove organic substances and fats can be used for metal degreasing before painting. But still, it is better to use complex compounds that transform rust into a useful layer and prevent its appearance in the future:
- white spirit;
- nitro solvents;
- degreaser on complex alcohols;
- kerosene.
As a degreaser is not recommended to use gasoline, as its effect on the surface appears invisible to the eye oily film, degrading the adhesion of the paint.
Degreasing must be done in a well-ventilated area with constant air circulation since the fumes from most of the chemicals used are very toxic. To avoid poisoning, it is recommended to wear a respirator, and work with rubber gloves and safety glasses – if any solvent gets in the eyes, chemical burns of the mucosa cannot be avoided.
Phosphating of metal surfaces
Phosphating is a process of coating surfaces of ferrous or non-ferrous metals with a thin film which protects them from rust formation and improves adhesion with the paint composition.
The application of such technology makes it possible to significantly improve the wear resistance of contacting parts in friction units. The method can be implemented for almost all alloys, except for high-alloy steel – a phosphate film of insufficient quality appears on it.
Why is phosphating performed?
Metal phosphating before painting is performed to provide the surface with reliable protection against corrosion processes in places that are mechanically cleaned from old paint and rust. Before applying a protective layer metal structures or products must be thoroughly cleaned from dust and dirt, as well as degreased.
This method of protection of metal structures allows their operation under conditions of:
- exposure to automotive oils and fuels;
- in electrical installations up to 1 kV;
- high humidity;
- in environments with organic solvents;
- finding under the paint coating.
The formed film can reliably protect the metal under the above conditions but is quickly destroyed in corrosive acid and alkaline environments. Therefore, before performing phosphating, it is necessary to determine the composition of the environment in which the metal product will be operated.
Phosphating methods
The formation of a phosphate protective film on the surface of the metal is obtained by several methods, the possibility and feasibility of which depend on the size of the structure and its application.
The most commonly used methods are as follows:
- treatment of the surface with special preparations, allowed even for mild steel, the result is the formation of a high-quality primer with anti-corrosive properties;
- use of phosphoric acid or “cold phosphating”, in which the thickness of protection is not more than 5 microns
- the use of zinc monophosphate, which is mainly used in the machine-building and electrical power sectors;
- treatment with phosphating paste.
To prepare the metal for painting, it is necessary to perform a number of compulsory procedures, without which it is impossible to quality painting and, accordingly, the long-term operation of metal structures.
Additional Questions
Is it necessary to prime metal before applying paint?
Priming metal surfaces before painting is crucial to eliminate the risk of rust seeping through your fresh coating of paint. Further, it aids in boosting the adhesion between the paint and the metal. In an event where some of the rust hasn’t been completely removed, using a zinc chromate primer will be helpful because it’s designed to coat rust.
How can you ensure that the metal is well-sealed before painting?
For a rust-free result, I always suggest that a rust-preventing primer, such as zinc chromate or iron oxide, be applied to the metal. If the metal in question has an uneven surface, a spray primer is usually your best bet. On the contrary, if you’re dealing with a smooth surface, a conventional metal primer, applied with a brush or roller, is the way to go.
Can I paint directly onto a metal surface?
Yes, but you should use paint that’s specially created for metal surfaces – you may find it labelled as either metal paint or multi-surface paint. Without this type of paint, you might find that your paint starts to flake, blister, or peel, ultimately destroying your finish.
Can vinegar be used as a cleaning agent for metal before painting?
Absolutely, white vinegar is not only a safe and effective way to clean metal surfaces before painting but also a more eco-friendly alternative to industrial solvents. All you need to do is soak a clean rag in white vinegar and then wipe down the metal surface. The acidity in vinegar will react with the metal to improve paint adhesion.
