Remember the DeLorean? Maybe you weren’t around for the 1980’s, but if you’ve seen “Back to the Future” you’ve seen a DeLorean. One of the coolest features of this car was the stainless steel body. (The flux capacitor didn’t come along until much later.) Not only did stainless look different, but it saved company founder John DeLorean the expense of putting in a paint shop. Other manufacturers have however stuck with painting their steel-bodied vehicles, despite all the work involved.
With your next fabrication project you do a DeLorean and insist on stainless. Or you could stay conventional and request a protective coating. There are pros and cons to both, but if you go the coating route you’ll want to know it will last. Here’s some advice on achieving a durable coating.
Understand the science
Fact is, not even Doc Brown really understands how paint sticks. The prevailing theories all center around three modes of adhesion: adsorption, chemical bonding and mechanical bonding. Adsorption is the stickiness that results whenever two surfaces come into contact. Chemical bonding is when atoms in one surface link to those in the second surface, and mechanical bonding is when the paint keys into a textured surface.
You don’t need to be a chemist to understand what matters though. If you want paint to stick it has to come into intimate contact with the metal beneath.
The 4 factors you need to know
Creating that intimate contact, and then ensuring the paint stays stuck, depends on these four points:
1. Structure condition
Is there already a coating or is this new metal? Existing paint should be removed to get a paint-to-metal bond, otherwise you’re bonding to old paint and you don’t know how well that’s adhering to the underlying metal. If you’re painting a new fabrication, what condition is the metal in? Sheet material could have oil on the surface while structural beams and tubes may be rusted.
2. Surface preparation and the application method
Paint has to flow over the surface rather than beading up like rain on a newly-waxed car. Achieving this means removing all the oil and grease. Sanding or bead blasting aren’t good methods because they’ll just spread the grease around. Instead, use a chemical cleanser or wash.
Surface texture is a second issue. A rough surface works better because it offers more surface area. That means more chemical and mechanical bonding. However, it’s not just a case of roughening things up. The profile of the resulting surface matters too. One that’s too rough could lead to pinpoint rusting but if it’s too smooth adhesion will be poor.
The main application methods are spraying and brushing. Spraying tends to build up a thick coating with good adhesion. Part geometry can be a problem though, in which case brushing is the answer. In addition, welds are often brushed before spraying.
3. Environmental conditions, during application and in-service
Humidity, temperature and sunlight are the biggest issues. High humidity affects adhesion while high temperatures can lead to paint drying too fast. Low temperatures are also a problem because the paint will dry slowly or not at all. Sunlight contains ultraviolet light that makes many colors fade and can cause cracking.
4. Paint quality
Paint is composed of pigments for color, a binder that provides adhesion, integrity and toughness, and a liquid carrier – water or solvent. High quality paint has a higher solids content that forms a thicker coating and incorporates an organosilane binder element that aids adhesion. As a consequence, higher quality paint is also more expensive.
Durability doesn’t come easily
Creating a long-lasting coating takes a lot of effort, but what’s the alternative? Don’t protect your fabrication and let corrosion eat it away, or go to a corrosion-resistant material like stainless steel? And don’t forget, painting gives you more appearance options while it’s hard work keeping stainless looking good. Next time you see a DeLorean, take a close look at the body and you’ll see what we mean.