Zinc: The best-kept secret in rust prevention
If you’ve got any kind of outdoor steel around your house that needs protection from rust, there are a few things you should know about the element zinc. It offers the most effective way to prevent rusting of all kinds of steel, and zinc is used like this in two ways. One of these ways is more widely known than the other, but they work together very well.
Steel is a mainstay of civilization because it combines strength and resiliency. Most of the outdoor metal in our world is steel, and everyone knows that steel rusts. Paint can prevent rusting for a while on wrought iron fences, gates, steel deck furniture and other things, but eventually all paint fails, allowing rusting to begin. That’s why a French man named Stanislas Sorel invented a process called hot dipped galvanization in 1836.
Unlike paint, the zinc used for hot-dipped galvanizing is bonded to steel on a molecular level that involves heat. The steel is first cleaned chemically, then dipped in a vat of moulten zinc. In the same way that solder bonds to copper pipes, the liquid zinc bonds to the steel right within the pores of the metal. This zinc never peels and renders steel completely corrosion-resistant for as long as the zinc coating remains — often permanently.
Hot-dipped galvanized steel is a medium grey colour, and there are two things to understand about it. First, galvanized surfaces don’t hold onto paint very well. They can be painted, but require a special primer. And even then, the paint is more likely to come off than on nongalvanized surfaces.
True hot-dipped galvanization is also a factory process that happens when items are manufactured. You can’t do it at home, and the thicker the zinc coating, the longer it protects against rust. The heaviest hot-dipped galvanized zinc coatings will protect against rust for longer than any of us will remain alive.
If you can find items you’re looking to buy that are hotdipped galvanized, they’ll last much longer than painted equivalents. But there’s a second way that zinc can protect steel and it’s quite DIY friendly.
Zinc protects steel against rusting even when it’s not been applied as hot-dipped moulten metal because it acts as an anode, and that’s where so-called “cold galvanizing” zinc spray coatings can help. Applied just like spray paint, it’s something you can do at home, and I know from experience that it works really well in two kinds of situations.
About 10 years ago, I had occasion to weld something onto hot-dipped galvanized steel, and heat from welding like this evaporates zinc coatings. I applied zinc-rich spray paint to the welded area after it had cooled and it has remained completely rust free even though it’s been outside the whole time. This is the same process you can follow for any previously galvanized area that needs re-protection, but there’s more.
Another way to use zinc-rich cold galvanizing spray paint is on bare steel items with no previous protection. This stuff has a medium grey colour (similar to hot-dipped galvanizing), and it can be painted over after the fact. Clean the metal thoroughly beforehand, apply one coat then add a second coat a few minutes later. Let the area dry for at least two days at room temperature, then apply primer and paint over top to match as you see fit. It’s a perfect approach for keeping anything made of steel looking great and rust free.
The best zinc-rich coating I’ve used is made by Rust-Oleum and it comes in a spray can or larger quart-sized containers for brush application on bigger areas. It’s 93% zinc, which is one of the reasons it works so well. For outdoor steel you care about, I don’t think you can do better.