Building owners expect that their assets will be protected by the specified coatings, but are often disappointed. Here is how to get it right.
When painting or coating a building, the obvious aim is for it to last as long as possible to protect the building against the elements and keep its aesthetic appeal. But sometimes coatings just don’t live up to expectations, whether it is because of incorrect specifications, poor surface preparation or coating failures.
Industrial coatings in particular are specified first and foremost for their protective properties, such as corrosion control, and it is therefore important that a coating should be specified and applied correctly.
Where does it go wrong?
Some of the most common problems which Simon Norton, consultant from Chemical Investigation Services, comes across when inspecting organic coatings such as epoxies or polyurethanes, are blistering and undercoating corrosion. When dealing with inorganic coatings such as aluzinc, hot-dip galvanising or electroplated zinc coatings, it is corrosion due to moisture and chlorides and ultimately the rusting of the underlying steel that he finds most often.
“Oddly enough, while surface preparation is often mentioned as the primary cause of coating problems, none of the coating failures I have yet looked at have been surface preparation related. Blistering can often be caused by surface contamination, but also for example by the non-release of gas created during the curing process,” he notes.
“Undercoating corrosion can occur in powder coatings where the permeable nature of the coating allows chlorides and moisture to get to the metal surface. In the case of galvanised or aluzinc materials, this starts a process of chemical reactions which lift the coating and speed up the material failure. Often the coating chosen for an application is inadequate for the stress and environment it is subjected to, which leads to the coating failing prematurely,” Norton explains.
“The proper choice of coating for a certain application, taking into consideration the environmental conditions under which the coating will perform, is key to preventing these problems,” he states.
Not all coatings are equal
One of the biggest mistakes, according to Norton, is the commodity approach to coatings where users are of the impression that all coatings are the same, when in fact formulations from different suppliers can differ radically in terms of performance as a result of the expertise and know-how propriety to the coating manufacturer.
“Recent testing of a new-to-the-market, high-performance industrial coating found that the product displayed unusual damage and deterioration features after a 1000 hour accelerated corrosion test, whereas other so-called inferior coatings passed the testing with flying colours,” he illustrates.
Dan Ancketill, Business Manager of Optima Coatings, adds that whether it is a new build to be coated or an asset that needs to be maintained, building owners, property managers, specifying professionals and project teams should involve a protective coatings consultant to get expert advice in order to ensure that suitable coating systems are specified.
“Also, to avoid specifications being overlooked or bypassed, the value chain needs to be monitored from initial specification to completed application,” he advises. “Incorrect application can also cause failures if, for example, the time between coats is stretched, which causes ineffective adhesion. The advisor from the coatings manufacturer should visit the site and follow up to verify that the coating system is applied as per the agreed specification.”
Selecting appropriate coatings
The first aspect to consider when specifying industrial coatings are the type of material to be protected and the condition of the substrate, according to Ancketill. In addition, the client’s requirements in terms of longevity of the coating system, as well as the environment to which the coating system will be exposed to, are important factors.
“One needs to determine the corrosiveness of the environment, especially where metal is involved, along with the degree of chemical and mechanical resistance that is required of the coating system and what kind of architectural finish the client requires whilst protecting the underlying substrate,” Ancketill highlights.
“The properties of the coating system in terms of ultraviolet (UV) stability and abrasion resistance – how many times people can walk on it, rub and scratch it or bump into it without it cracking, as well as its ability to move with the substrate – also play a role,” he adds.
Specifying cheat sheet:
1. Identify the type and condition of the substrate.
2. Determine the required longevity of the coating.
– Consider the client’s tolerance to changes in colour or feel.
3. Evaluate the environment.
– Corrosiveness of the area.
– Required chemical resistance of the coating.
– UV stability of the coating.
– Abrasion resistance of the coating.
– Architectural decorative finish of the coating system.
4. Involve a coatings consultant.
5. Include stipulations for surface preparation.
Investigating existing coatings
In the case when a substrate is already coated, one needs to identify whether there are any failures. If so, these need to be investigated to find the cause.
According to Norton, when investigating a coating failure, it is critical to acquire the entire history of the coating, the make and generic type of coating and the product data sheets. “This includes understanding the application of the coating, the surface preparation and what records exist to prove the preparation was in fact done.
“Next, a short literature review will clarify the ins and outs of the specific coating that was used in the case of the failure, which would lead to a better understanding of the coating’s chemistry and whether the coating type and chemistry can be linked with the apparent coating problem,” Norton explains.
Following this preparatory work, the problem should be inspected on site, where on-site personnel involved with the complaint and the application of the coating should be interviewed. These steps will allow the investigator to develop one or more hypotheses about the possible cause of the coating failure or problem. Based on these hypotheses, sampling and testing of the coating, the surrounding environment and, in some cases, the substrate are important to ultimately determine the cause.
“The key to these investigations is not to be led by theories advanced by the coating supplier, coating applicator or the client, as these may well lead to a wild goose chase to nowhere,” Norton warns.
Preparing the substrate
Once the condition of the substrate has been evaluated, before a new coating can be applied, it needs to be properly prepared.
Ancketill explains that if the substrate is not cleaned correctly or if the existing coating is not taken off completely, the new coating can pull off the previous one and not adhere to the surface as it should.
According to Ancketill, one of the most popular methods of preparing a steel surface is to use a high-pressure blasting methodology involving sand or grit to blast off an existing coating and achieve an abrasive blasting grade of Sa 2.5 according to ISO 8501-1.
ISO 8501-1 Surface Finish Grade Sa 2.5
After surface cleaning and washing to remove grease, oil and chlorides the substrate is blasted to Sa 2.5 which leaves the surface free from poorly adhering mill scale, rust, paint coatings and adhering solids.
“Alternatively, if blasting is not possible, a mechanical cleaning method is followed. Firstly, the piece of metal is degreased, after which all rust and loose paint are removed with wire brushes and sandpaper. Then the steel is wiped or washed until a water break-free substrate is achieved,” Ancketill explains.
Specifying substrate preparation
Norton adds that the substrate preparation also needs to be described in detail to contractors, and the coating required should be described by brand name or generic type with some recommended brand names.
“If unsure, then the best is to seek the independent advice of a corrosion specialist or consultant who can fully evaluate the requirements and may even recommend some accelerated testing of the coating on selected materials before onsite coating is carried out,” he recommends.
“Another suggestion is to install corrosion coupons that are coated and uncoated at the site, and include other coating types so that should the preferred coating fail or deteriorate, then there is a proven option to use.”
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The future of coatings
According to Ancketill, while using lead-free coatings has now become the norm, the next major trend is a move towards solvent-free coatings and water-based coatings.
“While solvent-free coatings are seen as the happy medium between solvent-based and water-based coatings, the latter has a whole range of benefits over the traditional solvent-based coatings since their carbon footprint, volatile organic compounds (VOCs) and environmental impact are much lower. Water-based coatings further have the advantage that one doesn’t need special protective equipment such as masks to apply the paint, and to clean the brushes no solvents are necessary, just water. Also, to make the paint thinner, water can be used.”
In terms of aesthetics, most industrial coating systems harness a range of different hues to match the RAL and NCIS colour charts.
Full thanks and acknowledgement are given to Chemical Investigation Services and Optima Coatings for the information given to write this article.