Ask Joe Powder Mar/Apr 2019
Posted on Monday, March 25, 2019
Blasted Cast Iron
Q: Joe, I’ve contacted you before so maybe you can help me again. I run a small to medium sized powder coating
line at a metal fabrication facility in southern New
Mexico. We have a five stage washer that utilizes
an alkaline first stage and a zirconium-based
4th stage. All stages use RO (reverse osmosis)
water. Stages 2, 3 and 5 are RO rinses. We have a
partitioned drying/curing oven, so adjusting one
without impacting the other isn’t possible.
I’m told we are going to be painting lots of
“ductile cast iron” parts in the near future. What
do I need to know about painting these types of
parts? I’ve read they can have outgassing and
adhesion issues. If true, how do I avoid such issues?
Management wants us to coat them with
“something” that will (I assume) fill in some of
the casting imperfections giving the final product
a more finished appearance. I’ve been told to find
out about some kind of thick primer? I’m told
we’d want a powder coating product that matches
one of the colors we currently use, but in a rough
texture that could “fill in” or hide the roughness
of the casting surface.
What do I need to do or apply to obtain the
Santa Teresa, NM
A: Hi Alan, Here’s where to start - blast the cast iron
with a decent media; I recommend medium grit
aluminum oxide. Then run the part through your
pretreatment system, including dry off. Next,
I would preheat the part to about 350 degrees
Fahrenheit, allow it to cool to about 225-250
degrees Fahrenheit, then apply a reasonably thick
coat (5 to 6 mils) of a good edge coverage (higher
viscosity) powder coating.
Your choice of chemistry will depend on the
service environment the part will experience and
the expected durability. If a single coat doesn’t
work, then lightly abrade the first coat and apply
a second coat of powder. You should undercure
the first coat if you need to use two coats. This will
ensure excellent intercoat adhesion.
Please let me know how things work out.
Q: Hi Joe, How can I repair a new bike rack that has a
black powder coating? It has a damaged area
about the size of a dime and is already bolted to
a large concrete base. We would like to make the
area rust proof. Is a patch the best bet or should a
paint be applied to protect it from the elements?
Thanks for your advice.
A: Hi Norm, Here is what I would suggest: Abrade and clean
the bare area, making sure that you remove all
oxidation, rust, oil, and dirt. I would abrade the
adjoining powder coated area as well. Next, apply
a phosphoric acid metal pretreatment per the
instructions provided by the supplier. You can find
these on Amazon and possibly at your local DIY
or auto parts store. This type of metal pretreatment
will retard corrosion and provide a bit of a chemical
anchor for your primer.
After the pretreatment dries sufficiently, mask
off the area to be coated and apply a good aerosol
epoxy primer. I would shy away from zinc-rich
types because they are difficult to apply. After the
primer dries, lightly abrade the primer with 400
grit automotive grade sandpaper and tack off the
sanding dust. Next, apply a suitable aerosol topcoat.
You can find these at a DIY or hardware store (if
there are any left in this world).
This coating system will not be as durable as the
original powder coating; however, if done carefully
per the supplier instructions, it should last a couple
years in a moderate climate.
The Answer is Yes, But...
Q: Is there a way to powder coat fiberglass?
A: Hello Nathan, I think you are inquiring about the possibility of
applying and curing powder coating over glass filled
composite, a.k.a. fiberglass reinforced plastic (FRP).
If this is the case, the answer is yes, but with a rather
specific process and materials.
You’ll first want to clean and lightly abrade the
surface to ensure good adhesion of the powder
coating. Most FRP harbors residual styrene which
will bleed out during the powder bake so it is wise
to prebake the substrate before applying the powder.
I would bake it to about 340 degrees Farenheit.
The next step involves applying the powder. You
may get lucky and be able to hit the hot substrate
with powder and get adequate film build. This is a
big maybe and requires timing and finesse. If that
doesn’t work you can apply a conductive solution
(I recommend checking with Chemical Technology,
Warren, MI), letting it flash and dry, then applying
the powder electrostatically as you normally would
That sounds pretty easy so far, but we’re not
done yet. You absolutely must use a very low
temperature cure powder and bake it no higher than
about 300 degrees Farenheit for the recommended
time. Not all powder manufacturers carry these
products, so you’ll have to look around. You can
use powdercoating.org to search for powder
manufacturers if you need a resource.
I hope this helps and please let me know if you
have any further questions/ideas.
Take a Bow
Q: Hey Joe, Are there established design guidelines for
through holes and threaded holes on powder
coated parts where plugging is not practical, and
tap/ream after powder would rather be avoided?
We manufacture Elite compound bows, etc. and
have in-house powder coating processes. Thank
A: Hey Mike, You guys make some awesome bows. I took a
look at your website - wow, very impressive. I think
with such a high end product you undoubtedly take
great care and time to make them perfect. I suggest
that you bite the bullet (or maybe the arrowhead?)
and consult with one of the high end masking/
plug suppliers on the Powder Coating Institute
website (www.powdercoating.org). These guys create
amazing products custom designed for your exact
application. I don’t think you’ll be sorry that you did.
If any of our erudite readers have another idea,
please let us know.
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