Substrates: A Summary

Kaser sees quite a variety of parts come through the shop – lawn furniture, cars, steel beams and everything in between. Despite the broad diversity of the parts we handle, the majority tends to fall into one of only three substrate categories: steel, aluminum, and stainless steel.

While we’re able to blast, pretreat, and powder coat all three substrates, we approach each one  differently to achieve the best possible results.

Steel

We begin by blasting steel parts with steel grit, creating an aggressive blast profile to maximize the powder coating’s adhesion later on. We then hang steel parts, and pretreat them with iron phosphate. If they’re going outside, we’ll apply a coat of epoxy primer first, followed by a super durable polyester top coat. The primer serves as an extra protective barrier between the substrate and moisture.  In certain applications we will skip the primer and apply two layers of super durable polyester top coat instead. If the part is meant for indoor application, one coat of regular polyester top coat does the job.

Whether we’re priming steel parts or applying two top coats, the goal is to build a film thickness that fully covers the part’s surface profile, including the peaks and valleys left behind by blasting with steel grit. A lot of powder coating shops don’t account for this – they’ll create a 2 mil blast profile with steel grit, and then apply 2 mils of powder coating, which adequately fills the valleys, but does not fully protect the peaks of the surface profile. It won’t be long before the customer is left with a prematurely rusty part. If you’ve experienced this frustration elsewhere, rest easy: at Kaser, we always account for the blast profile when determining how thick to build our powder coating film.

Aluminum

We blast all aluminum parts with crushed glass. Steel grit is too aggressive for aluminum – it would impregnate steel grit in the substrate, eventually creating tiny pinpricks of rust upon exposure to moisture. Crushed glass, on the other hand, gives us the blast profile we need without any risk of impregnation.

We then hang aluminum parts for pretreatment. Unlike steel, we don’t use iron phosphate, as iron phosphate is unable to build on nonferrous metals. Instead, we start with an acid etch. The acid etch enhances the surface profile on aluminum parts, increasing the powder coating’s ability to adhere. After rinsing off the acid etch, we pretreat the part with zirconium. Zirconium creates a barrier against moisture, which limits the chances of oxidizing, extends the life of the part, and safeguards against powder coating failure. Rust may not be an issue for aluminum parts, but we still want to protect them against moisture so they don’t oxidize.

Stainless Steel  

Stainless steel is a harder substrate than aluminum, but the same principles apply: we blast with crushed glass to avoid the possibility of impregnation. We then treat the part with an acid etch, rinse it, and apply zirconium pretreatment.

Aluminum and stainless substrates aren’t as vulnerable to corrosion, so there’s less of a need to prime them. If we prime them at all, we’ll use an epoxy primer. Since crushed glass creates a milder blast profile than steel grit (1 – 1.5 mils instead of 2 mils), we don’t have to build as much film thickness as we would on a steel part. More often than not, we’ll skip primer altogether and simply apply a super durable polyester top coat (or two).

If we apply a second layer of powder coating to aluminum and stainless parts, it’s most likely because there’s been some outgassing – a release of gas during the curing process that creates tiny, pinprick holes, particularly visible in glossy coating. A second layer of powder coating not only fixes the aesthetic issue, but fully protects the part.

Whether we’re dealing with steel, aluminum, or stainless, the goal is always the same: protect the part as fully as possible, for results that last as long as possible. The substrate material dictates how we go about that, and the degree of blast profile dictates how much film thickness to build during the powder coating process. If you’ve had problems with powder coating in the past, it’s likely because the shop wasn’t responsive to the substrate’s needs, or the depth of the blast profile. Give us a shot – you’ll be pleased with the results.

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