ASK JOE POWDER
The “Ask an Expert” Column Kevin Biller
kbiller@chemquest.com
I would love to be able to do the coating etc. in-house with the most affordable but lasting option. I’d like to lose the zinc plating all together because this is, in my opinion the root of all my evils. If I were to just powder coat, what in your opinion would be the best cost to quality option? I’ve been looking up thermoplastic coating with great interest. Please advise on surface preparation too. I’d like to cut out as many service providers as possible and have greater control and certainty regarding the accuracy the coating process. Perhaps your solution may be worth spending on a small home setup if it’s affordable. I make standard size door gates which are installed both indoors and outdoors. From indoor up to 100 km from the coast to outdoor 100m from the sea. Kind regards, Barry Lottering Port Elizabeth, South Africa The two years of our “Ask an Expert” column end with increasingly challenging questions about the powder coating. And this time too our Joe Powder does
Hi Barry,
not disappoint us with his always punctual and detailed answers.
Thanks for the question. I took a look at your website and, wow, you do amazing work. You turn simple gates and railings into exquisite artistry. I think you probably already know the source of the inconsistency in
Hi Joe,
your coatings, it’s the zinc plating. Undoubtedly it is wise to use a two-
I came across your blog whilst looking for solutions to an
coat process to ensure a good combination of corrosion resistance and
ongoing problem I’ve had. I design and manufacture designer
outdoor durability. This is definitely a must for hardware that has to
gates cut from 3mm mild steel. When I get the raw steel back
endure a coastal environment. But zinc plating can be inconsistent.
from laser cutting it is still covered with mill scale. I install all of
Here is what I recommend. Take the finishing process in-house. This will
my gates in a coastal region and because my gates are bespoke
involve not only coating process, but also the metal preparation as well.
boutique pieces I’d like them to last as long as possible while
Since you are laser cutting some of the ware and the parts have residual
exposed to the elements.
mill scale you should blast them with a decent media to lose the mill
I’ve been zinc plating and powder coating them with various
scale and clean the laser cut edges. I suggest aluminum oxide. Next step
service providers but this is hit and miss. It’s incredibly
is a three stage phosphate pretreatment. Google “metal pretreatment
frustrating to hear the powder coater call and say the powder
companies” in South Africa and ask them for recommendations. After
has come out of the oven full of pinholes and usually they say
pretreatment, thoroughly dry the parts with a convection type oven. The
they have no idea what happened. Worse still the zinc plater
air movement will help to completely dry the metal surface.
will also claim ignorance. I’ve had good results but it’s touch and
The next step is to completely cover the metal with an epoxy powder
go and when things go wrong it’s very expensive to fix and I’m
coating primer. If you have tight areas inside of channels, you should
usually left carrying the cost. I’ve tried everything from baking
pre-heat the metal and spray those areas whilst it is still hot. After
before coating to cleaning the gates before with degreaser and
the parts are completely coated, bake the primer for about 5-10
etching solutions, leaving them for a few days to off gas. I also
minutes (see the supplier’s technical data sheet for the correct
have some channels in the gate and where the radius is tight the
temperature). Make sure the coating has flowed out adequately to form
zinc doesn’t deposit too well and often I prime it which is time
a smooth surface. The 5-10 minutes should represent a deliberate
consuming and I’m never sure whether this is good to powder
underbake. This provides optimum adhesion of the next coat. After
coat over.
the part has cooled, lightly sand the primer with a 220 grit sandpaper.
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N. 84 - NOVEMBER/DECEMBER 2023 - international PAINT&COATING magazine