Pretreatment for powder coating

Why add pretreatment to architectural Aluminum? Adding performance coating to architectural aluminum – be it powder or liquid – is important for protection, durability, aesthetics…the list goes on. But what do you know about the preparation of the metal before the coating goes on? Perhaps just what it says in the spec, “pretreat and prepare as per manufacturers instructions”? Check out this short explanation of what happens beneath the coating – and why it’s important!

The 30 second elevator explanation - what does pretreatment involve?

We clean the aluminum, apply a chemical wash/conversion coating to it, dry it off, and it’s ready to have the coating applied. Simple.

Why do we need to do that?

We need good pretreat for two reasons – and both are super important!

1. Adhesion – good pretreatment helps the coatings to stick to the part.
2. Corrosion protection – good pretreatment really helps deliver corrosion protection, which is so important for anything exterior.

Ok, that’s the high level explanation. Want a little more detail (and some sustainability advantages to boot)? Lets not get too detailed, but here’s a little more explanation.

We have to clean the metal and remove any oil, dirt or grease that may have built up on the substrate, and aluminum needs to be etched to remove any oxide layers. If we don’t remove that, we just end up with powder coated, oil, dirt and grease. And that effects the powder performance, but it also affects the substrate performance.  The cleaning part is done with water of a specific pH level – think of it like a jet wash!

Applying the pretreatment – this is where the chemical wash, the actual pretreat solution, is applied to the substrate and is needed for the reasons outlined above. This stage will create and inert film that promotes adhesion and corrosion resistance. This is where there are some differences between powder and liquid coatings.

To get a warranty in the USA, performance liquid coatings nearly always require a chrome based pretreat – at least at the 2605, Fluoropolymer, superior performance level.

Chrome makes a great pretreat, but it’s not great for the environment, and it’s not that great for people.

Equivalent powder coatings can be applied over chrome pretreat, but can also be applied over non-chrome pretreat and still achieve those top levels of superior performance. So, it’s nice that there is the option to remove chrome from the environment and chrome from people. Sustainability advantage!

After the pretreatment solution is applied, if it is a chrome based pretreat then a rinse occurs and then a sealant is applied and finally the part goes into the dry off oven.

You can probably guess what happens in the dry off oven! When the pretreated part comes out of the dry-off oven it’s ready to have the coating applied.

And this is where another difference with liquid performance paints comes in. Liquid performance paints always require a primer. And it’s a chrome-based primer, before 2^nd and sometimes 3^rd or 4^th coat are applied (depending on geography, effect etc.).

There are two reasons why a primer IS needed with liquid systems:

1. Adhesion – liquid adhesive qualities aren’t as good, so the primer is added to stick to the substrate, top-coat sticks to the primer, and they work together to deliver the adhesion we want.
2. The PVDF fluoropolymer resin used in 2605 liquid performance coatings actually allows a little bit of light through the surface of the coating, so we want the primer down there to help protect against UV degradation.

With powder coatings, those two reasons to require a primer don’t exist. Sustainability advantage!

Everything needed for adhesion in powder coatings (aside from the aforementioned pretreatment) is extruded into the powder when it’s manufactured. So adhesion is basically built in!

Likewise, 2605 performance powders do not use PVDF fluoropolymer resin, but FEVE fluoropolymer resin. And FEVE does not allow light through. So not primer needed there either.

So 99% of the time, powder is a single coat application on aluminum (and yes, that includes with mica/metallic effects. You don’t have to add a clearcoat to the top as we make our metallic/mica sparkle coatings differently too, using encapsulated mica/metallic – this means it’s encapsulated in resin which is the stuff that does the hard UV protection work in a coating - and then a process called bonding, which makes each individual particle of powder stick to a particle of mica/metallic. The mica/metallic is therefore protected by the resin in the coating as well!).

Are all pretreatment systems made the same?

The short answer is no. There can be wand wash systems, 3 stage systems, 5 stage systems, 8 stage systems…the list goes on. Some coaters won’t have a chemical pretreat and will use mechanical instead. The system in use will depend on the desired quality level offered to the final customer.

This is where for major projects, or anything that you want a warranty on, using Registered Applicators is the way forward. The applicators go through strict audits, and a big part of that audit is their pretreatment system – plus the application line and oven. If good pretreat is the path to good adhesion and good corrosion protection, it’s important. Making sure the coater is set up for success is equally important, and Registered Applicators have proven that.