Powder coating aluminum for outdoors and UV resistance

A. Hi Marc,

From your reply, I take it that you are not currently applying a chromate conversion coating on your work. The chromate will provide big improvements in corrosion resistance.
As far as your powder coating goes, you should go back to your supplier and see whether he has a more robust powder coating product for outdoor applications.

With a little bit of comparison testing, you should be able to see whether you need to change the pretreatment, the paint, or both.

A. There also exist proprietary pretreatment processes which will provide high salt spray hours on aluminum without involving chromates.

Chromate Conversion + Polyester TGIC = Happy Hussey Guy.

Initially, you should find out what type of powder you are using. An epoxy powder is the worst type of powder that you can use outdoors. Polyurethane is the best type of powder to use outdoors. Of course a primer application will help with corrosion. Obviously more expensive as well.

Hope this helps. I will be doing more testing and will try to note my results soon...

A. Hi, Russell. If you search the site for "preheat powder" you will see a number of discussions on this subject. You will also see that some experts feel that preheating is a bad idea as it is wasteful of powder and, in their opinion, makes a perfect finish difficult.

Regards,

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Q. Does anyone have experience with heating parts up (with convection or IR) and then applying powdercoat? The part heat is what we're thinking will properly gel the part.

Immediately after this, we'd apply a finish coat of powdercoat and finally the parts would go through an oven to cure.

Is this following accepted principles?
What issues would we see?
What variables does this force at us?

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Q. Is preheating the part to be powder coated necessary? Does it help or not to preheat to remove impurities? what does it do to the zinc phosphate coating if the piece is preheated?

A. Your painter should be using polyester/polyurethane TGIC. There might be a problem with getting the exact look you want with the resiliency that you are looking for, so he might be giving you the best product that he can find that will meet the spec's(probably a hybrid). Most paint manufacturers only give a one year guarantee against fading and chalking in the field. Sheldon Taylor supply chain electronics Wake Forest, North Carolina A. Polyester powder should do much better than you're getting. If you were using epoxy you might expect chalking in UV exposure. You don't say what you are using. Try polyester if you're not using it. If you are find a new supplier. Geoff Crowley Crithwood Ltd. Westfield, Scotland, UK

A. Dear Norm,
I do not think that many people realise that there are industrial polyesters and architectural quality types known as "Super Durable Polyesters". However, you do state that the atmosphere has a very high salt content to which we have the added factor of weathering (UV). I hate to say it but in my opinion there is only one finish that will withstand a combination of high UV and salt attack; that is a polyvinylidene fluoride coating with suitable primer. Aluminium profiles correctly pretreated and coated with this product will result in over 3000-4000 hour's resistance to salt and UV weathering laboratory testing. With your specification, I am sorry to say the thermoplastic chemistry beats the thermosetting one.

A. Hi, Thomas. The powder coating shop will normally do the pretreatment. Ideally it would include chromate conversion coating pretreatment before the powder coating as this is the best. As a consumer you will probably have a hard time buying chromate chemicals for your own use because hexavalent chromate requires careful stewardship. Good luck.

Regards,