Painting Metal Surfaces
By Dave Weaver, PCM Paint Division Manager | May 16, 2017
When is the best time of the year to paint my Mansard roof?
The optimal time of the year is when the humidity is low and the air temps are under 80 degrees, so early spring and early fall. Once the air temperatures reach 90 degrees or above you have to figure the roof substrate itself can reach temperature in excess of 115 degrees when baking in the sun all afternoon. The problem with that is the coatings dry extremely quickly from the outside in which can cause several problems:
- The roof does not dry evenly so there is not a uniform finish.
- The coating dries from the outside in, trapping the gases as the coating dries. As the temperatures drop, the gases contract. This is not a problem but as the temperatures rise again, the gases trapped inside the coating film and expand causing what is called “Off Gassing” and ultimately causes the coating to blister and peel.
Painting metal surfaces can be tricky when you don’t know the makeup of the metal, how it should be primed, or what type of paint to use. That’s why we are sharing some tricks of the trade – in hopes of making it a little easier for you.
First, you should know there are two categories of metal – ferrous (containing iron) and non-ferrous (not containing iron). How do you know which is which? The easiest way to distinguish a ferrous metal from a non-ferrous metal is with a magnet. If the magnet sticks to the metal, it is ferrous. If not, it is non-ferrous.
Why do I need to know the type of metal that is on my property?
It is important to identify ferrous metals because they are known to rust. While ferrous metal (steel) often arrives at the jobsite pre-primed with a factory primer, this coating will not prevent rusting. Metal corrodes when it is exposed to the atmosphere. Some metals, such as stainless steel, form an oxide layer that is very stable and protective, while other metals, such as zinc, corrode to produce a weak non-protective layer.
Knowing which type of metal you’re working with is vital, since it will impact the performance of the coatings that you select for the job. Coating the metal substrate with a primer and paint will provide barrier protection by isolating the metal from the environment.
What is galvanized metal?
Galvanized metal is ferrous metal coated with a layer of zinc. The value of galvanizing stems from the corrosion resistance of zinc, which, under most service conditions, is considerably greater than that of iron and steel.
Painting galvanized metal can prove to be difficult and demands regular maintenance. Surface preparation is a key factor in achieving good paint adhesion on a galvanized surface. When galvanized metal is used on exterior surfaces, the oily film will weather away after six months, allowing the surface to be painted with minimum preparation (simply removing any dust or dirt). However, when galvanized metal is used on interior surfaces, protected from the wear and tear of nature, the surface oil should be removed manually before priming and painting. After the galvanized metal is cleaned, it is a good candidate for DTM coatings.
Speaking of DTM coatings, what are they?
DTM, or “direct-to-metal,” coatings are paints composed of half-primer, half-topcoat. DTM paints allow painters to bypass the primer and dive right into applying the topcoat. A DTM coating is a latex-based paint that does not prevent against rusting. If you need an exterior surface painted, you will most likely use oil-based paint, commonly referred to as “industrial alkyd enamel paint.”
What about bare metal?
Bare metal is untreated ferrous metal that must be covered with a rust deterrent. If rust is already present, it must be removed prior to re-coating. This can be done by scraping the surface with a brush, sanding, abrasive blasting, or dissolving the rust away with acidic solutions. All new ferrous metal surfaces should be primed with a rust inhibitor prior to finish coat application. While primers are designed to offer an initial protective coat and provide a base for the finish coat, it is the finish coat that delivers the extra layer of protection against moisture and adds aesthetic appeal.
Sample scope for ferrous metal:
- Sand and scrape substrate to remove rust and any failing paint
- Spot prime bare or rusted spots on metal with a rust inhibitive primer
- Furnish and install 1-2 coats (top coat) depending on what substrate it is
If you find yourself painting previously painted or rusted metal, it is crucial you remember to prep it. First, sand the old paint and rust off of the surface. If it’s really bad, you may try a wire brush and grind the remnants off the failing metal. While you may not be able to remove every inch of the existing rust, we recommend you use a rust inhibitor primer on all bare metal and/or existing rust that remains. This will prevent the remaining rust from bleeding back through the new paint job. After that, finish off the job with 2 coats of oil-based paint.
Now does getting that metal railing re-painted seem so daunting?