Painting metal surfaces using a spray gun is a process that requires not only high-quality equipment, but also strict adherence to technology. The slightest deviation from the rules of preparation or application of the material often leads to the formation of unaesthetic streaks, sagging and “shagreen skin”. For a novice body shop or DIYer, the task of getting a perfectly smooth mirror finish may seem impossible, but understanding the physics of spraying makes a world of difference.
The main reason for the appearance of defects lies in the imbalance between the amount of paint supplied, air pressure and hand speed. When excess solvent is applied to the surface or the layer is applied too slowly, gravity causes the material to flow down, forming characteristic tears. To avoid this, it is necessary to understand in detail the preparation of the instrument, setting the viscosity of the enamel and the correct technique of movements.
In this article, we will go into detail about each stage of the process, from solvent selection to final polishing. You will learn how to set up spray torch for a specific part, why the temperature in the paint booth is critically important and how to avoid typical mistakes that even experienced craftsmen make when in a hurry. The right approach will allow you to get a result indistinguishable from the factory coating.
Metal surface preparation and selection of materials
The quality of the finishing coating depends 80% on how well the preliminary preparation of the metal was carried out. Before picking up the spray gun, the surface must be absolutely clean, grease-free and smooth. Any remaining unevenness, dust or traces of oil will become centers of crystallization of defects that will appear immediately after the first coat has dried.
The process begins with thorough washing and subsequent degreasing with special compounds. It is important to use exactly automotive degreasers, since household chemicals can leave a film on the metal that is invisible to the eye and impairs adhesion. After treatment, the surface is wiped with an antistatic cloth to remove the smallest lint and dust that may get into the paint.
Particular attention should be paid to the choice of paint and varnish material and its compatibility with the primer. For metal surfaces, acrylic enamels, metallic base paints or nitro enamels are most often used. Each type of material requires a different approach to dilution and drying. An incorrectly selected solvent can cause the paint to boil or, conversely, take too long to dry, increasing the risk of dust settling.
⚠️ Caution: Never skip the primer or seal coat step when working on problematic metals such as aluminum or galvanized steel. Direct contact of some enamels with these surfaces can cause a rejection reaction that cannot be corrected by polishing.
To achieve the best results, it is recommended to use a material system from one manufacturer. This guarantees the chemical compatibility of all components: primer, base and varnish. If you are using products from different brands, be sure to consult the data sheets or perform a compatibility test on an inconspicuous area of metal.
Setting up the spray gun: pressure, torch and feed
Proper adjustment of the sprayer is a key point that determines whether the paint will lie smoothly or will ripple and drip. Modern spray gun systems HVLP (High Volume Low Pressure) require precise adjustment of three main parameters: torch shape, material supply and inlet pressure. Ignoring at least one of them will negate all efforts to prepare the surface.
The first step is to adjust the shape of the torch. To paint large flat metal surfaces, the torch is opened as wide as possible to ensure even overlap of the stripes. However, when working with dense reliefs or narrow parts, the torch should be narrowed in order to control the paint getting into the desired area and to avoid fogging.
Air pressure is the second critical parameter. Too high a pressure will lead to a significant overconsumption of material and the formation of “shagreen skin” (orange peel), since drops of paint will not have time to spread. Low pressure, on the contrary, will produce large drops that will definitely flow. The optimal value is usually 2-2.5 atmospheres at the nozzle exit, but the exact numbers depend on the viscosity of the paint.
How to check pressure without a pressure gauge?
If your spray gun does not have a built-in pressure gauge on the handle, you can use the test spray method on cardboard. If the paint comes out in large splashes and forms “blots”, there is little pressure. If a dry, rough surface with a lot of fog around is formed, the pressure is excessive. The ideal mark should be smooth and shiny immediately after application.
The material feed is regulated by a screw that limits the needle stroke. Beginners are recommended to open the feed not completely, but by 2/3 or 3/4 of a turn, increasing the flow only as they gain experience. This will allow you to control the amount of paint that gets onto the metal and minimize the risk of drips when your hand moves slower.
Preparation of working solution and viscosity control
One of the most common causes of drips is an incorrectly prepared working mixture. Paint, like any technical product, has strictly defined viscosity parameters at which it is sprayed best. Using paint “by eye” without measuring viscosity with a viscometer is a guaranteed path to failure.
The viscosity of a material directly affects its fluidity. Paint that is too thick will not have time to spread and will form roughness, while paint that is too thin will instantly flow down under the influence of gravity. To measure this parameter, a funnel-shaped device is used - a viscometer, which allows you to measure the time it takes for a certain volume of liquid to flow out.
The process of preparing the mixture requires precision. It is necessary to mix the base and solvent in the proportions specified in the technical data sheet (TDS) of the manufacturer. Typically this ratio is 10:1 or 4:1, but it can vary depending on the ambient temperature and type of paint.
| Material type | Recommended viscosity (sec) | Nozzle size (mm) | Pressure (Bar) |
|---|---|---|---|
| Soil filler | 18-22 | 1.6 - 1.8 | 3.0 - 4.0 |
| Base enamel (metallic) | 16-18 | 1.3 - 1.4 | 2.0 - 2.5 |
| Acrylic enamel (solid) | 18-20 | 1.4 - 1.5 | 2.5 - 3.0 |
| Varnish (2K) | 16-18 | 1.3 - 1.5 | 2.0 - 2.5 |
Cool paint is thicker, and if you add more solvent to thin it, you can upset the chemical balance and end up with dullness or a change in shade. It is better to pre-warm the material to room temperature (20°C) before mixing.
Use only solvents recommended by the manufacturer. A fast-drying solvent (summer) may not have time to spread, and a slow-drying one (winter) may cause drips. Select the type of solvent strictly according to the season and temperature in the paint shop.
Application technique: distance, angle and speed
Even a perfectly tuned tool will not give results without the correct application technique. The movements of the master’s hand must be smooth, uniform and parallel to the surface of the part being painted. Any change in the distance or angle of the spray gun instantly affects the thickness of the layer and the uniformity of the coating.
The optimal distance from the nozzle to the metal is 15-20 centimeters. If you bring the tool closer, the concentration of paint will be too high, which will inevitably lead to drips. As the distance increases, some of the paint will evaporate in the form of fog before reaching the target, and the layer will turn out dry and matte.
The spray gun should always be perpendicular (at an angle of 90 degrees) to the surface to be painted. Tilt of the tool to the left or right leads to an uneven distribution of the material: on one side there will be more layers (risk of drip), on the other - less (risk of unpainting). This is especially critical when working with convex body shapes.
The speed of the hand movement must be constant. The standard recommendation is about 40-50 cm per second. You need to move the spray gun jerkily, starting the movement even before pressing the trigger and releasing it only after passing the edge of the part. This ensures uniform layer thickness over the entire area.
Painting algorithm: overlap and number of layers
The process of painting metal is based on applying several thin layers, rather than one thick one. Trying to cover up the color or hide soil defects the first time with a “fat” layer is the main mistake of beginners, leading to the formation of “tears”. Technology requires patience and consistency.
The first layer is often called the “dust” or “adhesion” layer. It is applied very thinly, from a distance of 25-30 cm, and does not require complete color coverage. Its task is to create a sticky base for subsequent layers. After it, the material should be allowed to dry (interlayer drying) for 5-10 minutes, depending on the type of paint.
The second and subsequent layers are applied with the obligatory overlap of the previous pass by 50%. This means that each new spray line should cover half of the previous one. This overlap ensures a streak-free finish and an even texture, especially important for metallics where the direction of the aluminum particles affects the final color.
☑️ Procedure for painting
The number of layers depends on the hiding power of the paint and color. Usually 2-3 layers of base and 2-3 layers of varnish are enough. Between each layer it is necessary to maintain the solvent evaporation time. If you apply the next coat too early, the solvent will be “trapped” inside, causing the varnish to boil or cloud in the future.
Elimination of defects and finishing
Even if all the rules are followed, sometimes minor defects may occur, such as shagreen, dust particles or micro-drips. It is important to be able to diagnose the problem and know how to fix it. Some defects can be corrected immediately, others require complete repainting after drying.
If a drip does form, do not under any circumstances try to wipe it with a rag or smooth it with your finger during the painting process. This will only aggravate the situation by introducing dirt and disrupting the structure of the layer. The paint should be allowed to dry completely, then carefully sand off the defect with P1000-P1500 abrasive and polish.
Final polishing is carried out after complete polymerization of the varnish (usually after 24 hours or after hot drying). Using a polishing machine with appropriate pastes allows you to remove fine shagreen and give the surface a deep gloss. However, the best polishing is its absence, that is, a high-quality applied layer that does not require modification.
⚠️ Attention: When sanding fresh varnish, be extremely careful. Do not rub the layer down to the metal or base as this will require local touch-up which may be noticeable. Always check the material removal depth.
To consolidate the result and protect the coating, it is recommended to use special polishes and waxes after final drying. They will fill micro-scratches and give the metal additional color depth and hydrophobic properties.
The secret to avoiding streaks is to apply 3 thin coats with proper exposure, rather than trying to cover the surface with one thick coat.
Frequently asked questions (FAQ)
Why did “orange peel” appear after painting?
Shagreen (orange peel) most often occurs due to too high paint viscosity, low pressure at the spray gun outlet or incorrect distance to the surface. The reason may also be the high temperature in the chamber, due to which the solvent evaporates faster than the paint has time to spread.
Is it possible to paint metal in high humidity?
Highly not recommended. High air humidity leads to moisture condensation on the metal surface and in the spray jet itself. This causes clouding of the varnish (a “whitening” effect) and can provoke corrosion under the paint layer in the future. Humidity should not exceed 65-70%.
Which nozzle is best for acrylic paint?
For most acrylic enamels and primers, the optimal nozzle size is 1.4 – 1.6 mm. For thick filler primers, 1.8 mm may be required, and for liquid metallic base enamels, 1.3 mm. The exact size is always indicated in the technical data sheet of the product.
Do I need to sand each coat of paint?
With modern wet-on-wet technology, sanding between coats of base or varnish is not required as long as drying times are followed. Only the primer is sanded before painting and, if necessary, the finished coating to remove defects.