I used to be a welder in a shop dedicated to custom-made ornamental wrought iron. It was a small company with two welders and a painter working out of a single space. After assembling a section, I would carry it by hand to the painter. While I was waiting for the welds to cool, I would watch the painter work.
He would first scrub down the steel with acetone to remove the mill coat. This is a coat of grease that foundries coat the steel with to keep it from rusting. Once that was done, he would give each piece two coats of red primer to give the paint something to stick to. A day later, once the primer had set, he would give each section a final coat of black paint. The day after that we would load them onto the truck and install them. One of the things that stood out to me was that, without fail, we would end up using foam brushes to touch up the paint where the red primer showed through. Paint isn't very good at sticking to metal, even when it is primed. It turns out that the best paint for metal railings isn't paint at all. Other coatings hold better, are more even in appearance, and last far longer when applied to metal.
Why Doesn’t Paint Stick to Metal?
Metals have a solid crystalline structure all the way through. This is what makes metal strong, and, for high-use buildings like apartments, it makes metal the best material for balcony railings. Steel does have microscopic irregularities, but these are confined to the surface. If you've heard the kitchen tale about how seasoning a cast iron pan requires heating it to open and close the pores of the pan, it's referring to these irregularities. They're not pores so much as they are microscopic pits and long crevasses that are the result of working the metal in a form. These pits don't provide much traction for paint to grip, but they can trap oils and water that will damage paint coatings.
Protecting Metal from Rust Can Affect Paint Adhesion
Like I mentioned at the beginning of this post, steel is given a rust-preventative coating to keep corrosion from destroying the steel before it reaches the fabricator. There are two common types of coatings used.
A mill coat is a thin layer of grease placed over the steel. This prevents rust during shipment and storage, but the grease does get absorbed into imperfections on the steel's surface, and the trapped grease can be released when the steel's temperature changes. Complete removal requires heat treating, or burning the grease off, and shops often don't have the facilities to bake large pieces before coating them with primer and paint.
Pre-galvanisation is a thin coating of zinc applied to the metal that protects the steel from rust. It's a very effective means of preventing corrosion, far more effective than a grease coating. It's also easier to paint, but when certain types of oil- and latex-based paints are used on galvanisation they will react with the zinc, causing saponification. This effectively creates soap and will cause the paint to come off.
Of the two options, a grease mill coating is more common among small shops. It's less expensive and it's easier for welders to work with. Galvanisation burns off when welded, which can cause a condition called metal fume fever that's unpleasant at best and lethal at worst. Welders tend to dislike galvanised steel for this reason. Moreover, burning off the zinc during welding also removes that rust-protectant layer, creating a place for rust to take hold in the final product. In fact, welding creates several issues that are usually ignored.
Welds Need Special Priming to Hold Paint and Resist Corrosion
Welding has several issues that are rarely dealt with. Grease from the mill coat can contaminate welds, creating porosity-or voids that go all the way through-in the weld. Zinc can also contaminate the weld in a similar way when it isn't completely burned off. Both processes can leave a path for moisture to work their way into the interior of the railing and generate rust. Welds are also a non-uniform surface with vast degrees of depth difference between different parts of the weld. This is the worst of both worlds, with the rough surface being too deep for paint to fill in fully. This lets moisture collect in the voids, causing rust.
There are two ways to prevent welds from rusting. The first is hot-dipped galvanisation. After welding, a workpiece is dipped into a bath of molten zinc to create or restore a rust-resistant coat of galvanisation. This is durable but has some disadvantages, one being that it doesn't usually fill all the crevices in the weld. Zinc and steel also create an electrical reaction when wet (and especially when salt is involved), causing a particular type of corrosion called galvanic corrosion in which the zinc begins rapidly bonding with oxygen, creating zinc oxide. This can cause a white powder to form which will damage the paint and other appearance coatings.
An electrophoretic coating, or e-coat, is better. It's a chemically non-reactive coating created by placing a piece of metal in a bath and running a charge through it. This causes the metal to strongly attract the coating into even the tiniest crevices, removing void spaces where moisture could collect. It leaves the piece well primed for paint or another coating type.
Powder Coating Is the Best “Paint” for Metal Railings
Powder coat is an appearance coating that comes in the form of a powder instead of a liquid. This powder has an electrical charge that causes it to stick to the workpiece. An interesting side effect of the electric charge is that applying too thick a coat to the metal is difficult, as the charged powder will repel excess. This helps ensure an even coat. Baked at high temperatures, the powder melts into a liquid, and when it cools, the piece is left with a smooth, even coating.
Powder coating has many advantages when compared to traditional painting techniques. The process takes place under controlled conditions, ensuring a strong bond with the metal. It is a flexible coating that expands and contracts with the metal piece it is bonded to, meaning that it won't crack and allow water or other substances to work between it and the metal's surface. The only real disadvantage of a powder coat is that it is impossible to touch up in the same way that paint can be touched up. However, a high-quality powder coat is unlikely to require touching up. In combination with an e-coating, a powder coating makes it extremely difficult for rust to get a foothold, making it possible to have railings that don't rust.
One company that treats their steel this way after welding is Fortress Railing Products. Not only are their commercial steel railing systems pre-galvanised, they also receive an e-coat beneath the powder coat, to fill in all the crevices of the welds for complete rust-proofing. It’s the kind of attention to detail that tells you that these products were built to look good for a long time. Fortress Building Products’ other product lines also provide long-lasting, low-maintenance solutions, from slip-resistant composite decking to sturdy steel fencing.