Posted 8 Apr 2022

The top 10 causes of contamination in automotive refinishing

Contamination is one of the scariest words among painters. Even when you think you have done a flawless job, there is always the risk of stains, halos, or craters appearing on the coated surface when the work is finished. There can be many reasons for this, including damage caused by careless preparation, poorly maintained equipment, and dirty workshop, as well as human error.
It is often difficult to determine the cause of a defect at a glance and, therefore, to apply the most suitable correction procedure. After having described the most common coating defects, in this article we will focus on the main contaminants that can come into play during the finishing phase.

Unwashed cars before finishing

Proper vehicle washing is essential to avoid introducing contaminants into the workshop and, more specifically, into the spray booth. Particular attention should be paid to the wheels, where the greatest amounts of dirt and debris typically accumulate.

Dirt and dust in the base coat or clear coat

Dirt and dust are among the main contaminants. Dirt, for example, can enter the wet film either through airborne contamination or because the base has not been properly filtered. Also, dirt may have been drawn into the clear coat and trapped. In these cases, you will see light or dark dots on the surface, depending on the color. 

To avoid this type of contamination, make sure the booth is always clean, check the crevices to remove dust, and wear appropriate work clothing (such as an anti-static and low linting suit).

If the dirt is in the base coat, the correction procedure is to remove the dirt particles and reapply the base coat to the damaged areas. On the other hand, if the dirt is on the surface of the clear coat, sanding and polishing might work as well.

Contaminated air

Air from outside can bring in dust, dirt, insects, and other contaminants that can affect the quality of the paint and painted parts. Air filtration is often taken for granted, but it is not always effective, and filters should be inspected and replaced regularly.

But even the air inside a workshop can be contaminated as a result of the work being carried out. For example, excessive sanding, carried out in open areas rather than in contained preparation zones, can cause dust to circulate throughout the workshop and into the spray booth. To limit this, we recommend that you always enter the booth with the car and engines running.

Contaminated washing instruments

The brushes used for washing or polishing can be contaminated by dirt, impurities, oils, etc., making the preparation of the car or the parts to be polished ineffective. As we mentioned early, impurities on the surface are among the most common causes of finishing defects. 
One of the possible consequences is the hologram effect, a defect of the bodywork that appears as a set of very fine micro-scratches joined together. For this reason, contaminated washing or polishing brushes must be promptly replaced.

Blisters caused by moisture

Excessive moisture absorbed by the lower layers can lead to unpleasant blisters on the bodywork. This can be caused by a variety of factors, such as residual sanding water in the corners or edges, extreme air humidity, or the use of water-based products that have not completely evaporated.

Poor spray gun maintenance or cleaning

Spray guns should be cleaned up after the application of each color or product. Dry paint left in the gun can come off and contaminate subsequent coatings. 
In addition to contamination problems, poorly maintained spray guns can lead to uneven surfaces, stains, streaks, or poor coverage during painting. Always check with the spray gun manufacturer for proper cleaning procedures.

Negative pressure in the spray booth

If a spray booth is subjected to negative pressure (i.e., when the air being expelled is greater than the air being drawn in), the booth itself can become a giant vacuum cleaner due to the chimney effect. External dust and dirt can be sucked in through any crack and end up in the booth. To keep contaminants out, booths should be balanced to operate at a slightly positive pressure compared to their surroundings.
Spray booth walls should be protected with special white non-woven adhesive films that protect the surfaces by capturing dirt and any overspray, keep the application area bright at all times and significantly reduce spray booth maintenance.

Human contamination

Inappropriate clothing represents one of the most common contamination risks. Most contaminants come from dirt that falls off clothing or skin.

To prevent contamination from affecting paint operations, painters should wear full anti-static suits, gloves, and masks. Some bodyshops are equipped with special preparation zones, installed just before entering the booth, where compressed air blowers remove any impurities from the overalls. In addition, many hair products, including shampoos, conditioners, and perfumes, can cause halos in the paint.
It should be remembered that all PPE should only be worn in the spray booth and mixing room.


If silicone is used in the workshop, for example in spray form as a lubricant, it can remain in the air for a long time and cause costly silicone contamination.
The presence of silicone in sanding and polishing dust can cause coating defects known as "crackings" that open up to reveal the substrate. The circular craters vary in diameter from 0.5mm to 3mm. They are usually visible during the application or immediately afterward.

Oil or grease

Another cause of cracking can be the presence of grease on the substrate or in the spray system. There are several products responsible for leaving oily residues on the surface, for example polishing agents and sprays.

In conclusion, many contaminations can simply be avoided through effective cleaning procedures of the parts to be painted, the system, or the working environment. Maintenance and error prevention can save a lot of time and money than having to resort to last-minute fixes.

More news