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Powder finish is oxidizing significantly every week

2005

I invented a new type of garden fencing and recently had to start having it made overseas (China). My new manufacturer has been fabulous and I can't believe I am having this problem now.
I received a sample panel about 3-4 months ago and immediately put it outside, as I always do, to see if the powder finishing is ok.( I had a horrible experience a couple years ago with a powder coater that used a indoor epoxy on a outdoor fence. It looked absolutely horrible after a week.)

We had a long period of rain and maybe saw 3-4 sunny days the first month I had it outside. The panel still looked ok, so I ordered 1,000 prepackaged panels, time was short and I had to.I thought from experience that if the finish is defective you will know right away after leaving it outside a few weeks.
I received the panels beginning of July and put some test panels outside about 5 weeks ago.
I checked the panel that I put out in May and it took me by surprise. The finish has dulled significantly. The 5 week panels are still a little shiny but you can tell which side has been exposed to the sun.

My biggest fear is that these 1,000 panels will eventually look like the first panel.

I took the panels to a local powder coater and he did a test with a cotton swab and said he got color (green) after a couple swipes. He says it wasn't cured properly or had defective/incorrect powder. He says he may be able to fix the problem by finishing off the curing process in the oven. Does anyone know if this will work? Does anyone have any other theories to why the powder finishing is oxidizing so quickly?

I have fencing that has been outside for 3 summers now and still has a high gloss finish. What kind of powder coat is this? If I know what it is called then I can request it.

Thanks !

Heather Nesic
garden fencing - New Hampton, New Hampshire

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Dear Heather,

I think you are very lucky to have nearby such a knowledgeable powder coater. He would have wiped the panel with a solvent ( MEK / methyl ethyl ketone or MIBK) and the swab indicated that under curing had taken place.
There are four powders that you should be aware of: Epoxy, Epoxy Polyester, Polyester and Acrylic. Epoxies have very good chemical resistance but as you have already experienced are not very weather resistant. Epoxy Polyester is classed as the workhorse of the coating industry. Most items you see indoors that are E/P powder coated. Polyester powders (and these can vary in quality) are called for where the item is destined for outside use Ø window frames, children's play furniture, garden furniture even automobiles are now being coated this way. Acrylics are possibly the ultimate in thermosetting powders, excellent resistance to yellowing and very good gloss retention. Unfortunately, acrylic powders are not always available and can be expensive.
We would suggest that you supply a specification stating that your items must be coated in a Polyester powder cured at 200 °C metal temperature. The metal substrate, prior to powder coating is thoroughly cleaned and most essential (to prevent rust creep at some future date) treated with a ZINC phosphate pre-treatment. Ask them to supply a guarantee that the coating will pass 1000 hours ASTM B117 -- this should give you 10 years plus and not just a couple of years. Request sample panels from your supplier to enable you to send the panels for independent testing (ASTM B117). This way you will have panels on file with date and batch reference for independent testing should you run into problems in the future.

Terry Hickling
Birmingham, United Kingdom
2005