Skip to content

What's the Difference Between Paint and Coatings?

By Wesley Crump

[Note that this article is a transcript of the video embedded above.]

There’s a popular myth that I’ve heard about several bridges (including the Golden Gate Bridge in San Francisco and the Forth Bridge in eastern Scotland) that they paint the structure continuously from end to end. Once they finish at one end, they just start back up on the other. It’s not exactly true (at least for any structures I’m familiar with), but if you drive over any steel bridges regularly, it might seem like the painting never quite ends. That’s because, despite its ease of fabrication, relatively low cost, and incredible strength, steel has a limitation that we’re all familiar with: rust. Steel corrodes when exposed to the elements, especially when the elements include salty sea air.

I’m doing a deep dive series into corrosion engineering. We’ve talked about the tremendous cost of rust and how different materials exhibit corrosion, we’ve talked about protecting against rust using dissimilar metals like zinc and aluminum, and now I want to show you the other major weapon in the fight against rust. If you’ve ever thought, “This channel is so good, he could make it interesting to watch paint dry…” well, let’s test it out. I have the rustomatic 3000 set up for another corrosion protection shootout, plus a bunch of other cool demos as well. I’m Grady and this is Practical Engineering. On today’s episode we’re talking about high performance coatings systems for corrosion protection.

You might have noticed a word missing from that episode headline: “paint.” Of course, paint and coatings get used interchangeably, even within the industry, but there is a general distinction between the two. The former has the sole purpose of decoration. For example, nearly everyone has painted the walls of a bedroom to improve the way it looks. Coatings, on the other hand, are used for protection. They look like paint on the surface, but their real purpose is to provide a physical barrier between the metal and the environment, reducing the chance that it will come into contact with oxygen and moisture that lead to corrosion. Combined with cathodic protection (that I covered in a previous video), a coating system properly applied and well maintained can extend the lifespan of a steel structure pretty much indefinitely. Although paint and coatings often include similar ingredients, are applied in the same way, and usually look the same in the end, there are some huge differences as well, the biggest one being the consequences if things go wrong.

There are definitely right ways and wrong ways to paint a bedroom, but generally, the risk of messing it up is pretty small. Sometimes the color is not quite right or the coverage isn’t perfect, but those are pretty easy to fix. In the worst scenario, it’s only a few hundred dollars and a couple of days’ work to completely redo it. Not true with a coating system on a major steel structure. Corrosion is the biggest threat to many types of infrastructure, and if the protection system fails, the structure can fail too. It’s not just money on the line, either. It’s also the environment and public safety. Pipelines can leak or break, and bridges can collapse. Finally, it’s often no simple matter to simply reapply a coating system because many structures are difficult to access and disruptive to shut down. Applying protective coatings is something you only want to do once every so often (ideally every 25 to 50 years for most types of infrastructure). That’s why the materials and methods used to apply them are so far beyond what we normally associate with painting and why the systems are often called “high-performance” coatings.

Let me show you what I mean. These are the standard US federal government specifications used in department of defense projects. We’re in Division 9, which is finishes, and if I scroll down, you can see we have a totally different document for paints and general coatings than the one used for high-performance coatings. There’s even a more detailed spec used for critical steel structures. If you take a peek into this specification, you’ll see that a significant portion of the work isn’t the coating application itself, but the preparation of the steel surface beforehand. It’s estimated that surface prep makes up around 70% of the cost of a coating system and that 80% of coating failures can be attributed to inadequate surface preparation. That’s why most coating projects on major steel structures start with abrasive blasting.

The process of shooting abrasive media through a hose at high pressure, often known as sandblasting, is usually the quickest and most cost efficient way to clean steel of surface rust, old coatings, dirt, and contaminants, and cleanliness is essential for good adhesion of the coating. But, abrasive blasting does more than just clean; It roughens. Most high performance coatings work best on steel that isn’t perfectly smooth. The roughness, also known as the surface profile, gives the coating additional surface area for stronger adhesion. In fact, let’s just take a look at a random product data sheet for a high-performance primer, and you can see right there that the manufacturer recommends blast cleaning with a profile of 1.5 mils. That means the difference between the major peaks and valleys along the surface should be around one and half thousandths of an inch or about 40 microns. It also means we need a way to measure that tiny distance in the field (in other words, without the help of scanning electron microscopy) to make sure that the steel is in the right condition for the best performance of the coating, and there are a few ways to do that.

One method uses a stylus with a sharp point that is drawn across the surface of the steel. The trace can be stored by a computer and the profile is the distance between the highest peak and lowest valley. Another option is just to use a depth micrometer with a sharp point that will project into the valleys to get a measure of the profile. Finally, you can use replica tape that has a layer of compressible foam. I have an example of several grit blasted surfaces here, and I can apply a strip of the replica tape. When I burnish the tape against the steel surface, the foam compresses to form an impression of the peaks and valleys. Here’s what that looks like in a cross-section view. When the tape is removed, we can measure its new thickness, subtract the thickness of the plastic liner, and get a measure of the surface profile. Here’s a look at how the foam looks after burnishing on a relatively smooth surface and a very rough one. I used my depth micrometer to measure a profile of about 1 mil or 25 microns for the smooth surface and about 2.5 mil or 63 microns on the rough one.

Just to demonstrate the importance of surface preparation, I’m going to do a little coating of my own here in my garage. I’ve got four samples of steel here: two I’ve roughened up using a flap disc on a grinder (in lieu of sand blasting), and two I’ve sanded to a fairly smooth surface. They aren’t mirror surfaces, but the surface profile is much lower than that of the roughened samples. I also have some oil and I’ll spread a thin coat on one of the rough samples and one of the smooth ones. I wiped the oil off with a paper towel, but no soap. So now we have all the phases of youth here: smooth and clean, rough and clean, rough and oily, and smooth and oily. I’ll coat one side of all four samples using this epoxy product, leaving the other sides exposed. Notice how the wet paint doesn’t even want to stick to the dirty surfaces, but it eventually does lay down. I put two coats on each sample, and now it’s into the rustomatic 3000, the silliest machine I’ve ever built. I go into more detail on this in the cathodic protection video if you want to learn more, but essentially it’s going to dip these samples in saltwater, let them dry, take a photo, and do it all over again roughly every 5 minutes to stress test these steel samples. We’ll leave it running for a few weeks and come back to see how the samples hold up against corrosion.

There are countless types of coating systems in use around the world to protect steel against corrosion. The chemistry and availability of new and more effective coatings continue to evolve, but there is somewhat of an industry standard system used in infrastructure projects that consists of three coats. The first coat, called the primer, is used to adhere strongly to the steel and provide the first layer of protection. Sometimes the primer coat includes particles of zinc metal. Just like using a zinc anode to provide cathodic protection, a zinc-rich prime coat can sacrifice itself to protect steel from corrosion if any moisture gets through. Next the midcoat provides the primary barrier to moisture and air. Epoxy is a popular choice because it adheres well and lasts a long time. Epoxy often comes in two parts that you have to mix together, like the product I used on those steel samples. But, epoxy has a major weakness: UV rays. So, most coating systems use a topcoat of polyurethane whose main purpose is to protect the epoxy midcoat from being damaged by the rays of the sun. It’s often clear to visible light, but ultraviolet light is blocked so it can’t damage the lower coats.

The coating manufacturer provides detailed instructions on how to apply each coating and under what environmental conditions it can be done. They’ve tested their products diligently and they don’t want to pay out warranties if something goes wrong, so coating manufacturers go to a lot of trouble to make sure contractors use each product correctly. They often have to wait for clear or cool days before coating to make sure each layer meets the specifications for humidity and temperature. Even the applied thickness of the product can affect a coating’s performance. A coating that is too thin may not provide enough of a barrier, and one that is too thick may shrink and crack. Manufacturers often give a minimum and maximum thickness of the coating, both before and after it dries. Wet film thickness can be measured using one of these little gauges. I just press it into the wet paint and I can see the highest thickness measurement that picked up some of the coating. Dry film thickness can also be measured in the field for quality control using a magnetic probe.

Of course, once the coating is applied and dry, it has to be inspected for coverage. Coatings are particularly vulnerable to damage since they are so thin, and defects (called holidays) can be hard to spot by eye. Holiday detecting devices are used by coating inspectors to make sure there are no uncovered areas of steel. Most of them work just like the game of operation, but with higher voltage and fancier probes. If any part of the probe touches bare metal, an alarm will sound, notifying the inspector of even the tiniest pinhole or air bubble in the coating so it can be repaired. Once the system passes the quality control check, the structure can be put into service with the confidence that it will be protected from corrosion for the next several decades to come.

Let’s check in on the rustomatic 3000 and see how the samples did. Surprisingly, you can’t see much difference in the time lapse view. I let these samples run for about 3 weeks, and the uncoated steel underwent much more corrosion than the coated area of each square. I also have dried salt deposits all over my shop now. But, the real difference was visible once the samples were cleaned up. I used a pressure washer to blast off some of the rust, and this was enough to remove the epoxy coating on all the samples except the rough and clean one. That sample took a little more effort to remove the coating. At first glance, the coating appears to have protected all the samples against this corrosion stress test, but if you look around the edges, the difference becomes obvious.

The rough and clean sample had the least intrusion of rust getting under the edges of the coating, and you can see that nearly the entire coated area is just as it was before the test. The smooth and clean sample had much more rust under the edges of the coating that you can see in these semicircular areas protruding into the coated area. Similarly, the roughened yet oily sample had those semicircular intrusions of rust all around the perimeter of the coated area. The smooth and dirty sample was, as expected, the worst of them all. Lots of corrosion got under the coating on all sides, including a huge area along nearly the entire bottom of the coated area. It’s not a laboratory test, but it is a conspicuous example of the importance of surface preparation when applying a coating for corrosion protection.

Like those samples, I’m just scratching the surface of high performance coating systems in this video. Even within the field of corrosion engineering, coatings are a major discipline with a large body of knowledge and expertise spread across engineers, chemists, inspectors, and coatings contractors, all to extend the lifespan and safety of our infrastructure.

Watch Video At: Practical Engineering.