“This is bad news, Hal.” The contractor looked up from his plans and walked over. Luke was crouching, inspecting the rivets and bolts at the base of the complex valve system that delivered chemicals from transport trains into the factory. Luke continued, “There’s corrosion all over this system. Normally, that might not be a big deal, but it looks like someone else kept painting over the damage instead of fixing it. Look at this!” Luke ran a magnet over the painted surface. At some points, the magnet held tight to the coating, but at other points, the magnet fell to the floor. Even through the top level looked well-painted and clean, the substrate underneath was telling a different story.

Hal protested, “Come on, Luke. It looks fine from here.” Luke continued his objection: “The metal under the paint is in bad shape and it’s likely getting worse. Maybe it holds up ten more years…or maybe it starts falling apart next week. If you insist on doing the work without addressing the underlying problem, then I suggest you find another painter. I’m not doing it.”

Sometimes rust under paint is easy to see, but most of the time it’s not.
Image courtesy of Instructables

At some point, every professional painter will encounter the effects of rust and corrosion in their work. Preparing a new metal or steel surface for paint is a simple series of steps. But what about metal surfaces that are already painted? What about discovering preexisting rust on oil rigs and used cars? If you aren’t completely removing all the paint on the substrate, then how are you making sure there’s no rust underneath? Are you taking great care to evaluate the substrate underneath for corrosion, degradation, and damage? Or are you letting the aesthetic appearance of the top-coat guide your judgement?

Unfortunately, there are no 100% reliable ways to test for corrosion under paint without destroying the object itself. The best way to test for rust and corrosion is actually to test the primary surface for anomalies. For example, a simple way to test for underlying rust is to run a magnet over the surface. Any area with significant rust or degradation will fail to attract the magnet with the same strength as non-rusting areas of the surface. Eddy current meters use electromagnetic currents to measure conductivity between ferrous materials. While eddy current meters are an excellent tool for testing conductivity and can help detect substrate anomalies, understanding the quantitative output of this tool simply indicates that something is different about the substrate below. The something could be rust…or something else entirely.

Interestingly, the United States military is at the forefront of research on corrosion and rust detection. The military possesses a literal armada of metal planes, boats, tanks, and industrial structures that tend to stay in service for decades at a time. Corrosion and rust are ongoing – and ever-growing – concerns for the Armed Services as they seek to keep assets combat-ready for long durations of time. Many energy companies (including those in the oil, gas, and nuclear power sector) are similarly committed to finding efficient ways to detect and fight rust. To date, their research has yielded some very useful tools for rust detection. Laser ultrasonic tools are able to penetrate and test substrates of significant thickness to detect structural weakness due to rust and corrosion. Unfortunately, this new advance in technology still requires great expense and often requires that the structure be disassembled and/or placed in a water bath for accurate results, but we’re hopeful that someday it’ll become more accessible to everyday professional painters.

Detecting existing rust and corrosion is a challenge because the tools and technology available for detection still aren’t mature. Despite a great deal of effort and investment by chemists, physicists, and other engineers, finding a way to accurately determine rust levels without destroying the substrate are near impossible. Industrial paint products can protect metal and steel structures, but only if used properly and applied to a surface free of previous damage. Turning the tide in you favor against rust and degradation doesn’t have to be hard but does require you addressing the problem correctly from start to finish, and that includes diligently looking for signs of rust before beginning your work. Click here to learn more about corrosion detection and prevention in industrial painting!