Ask Joe Powder Jan-Feb 18
Posted on Monday, January 29, 2018
Path of Least Resistance
Q: Dear Joe,
Aluminum vs. steel. The coatings are
coming out different. The aluminum has a
thinner mil build than the steel using the
same manual gun. Any thoughts we could
give to our customer?
Steven, Syracuse, N.Y.
A: Hi Steven,
This is an interesting question. Theoretically,
there is no reason for powder to deposit
differently on steel vs. aluminum. Both metals
are conductive enough to allow an even
deposition of powder. Hence, this begs a few
questions. As we both know, film thickness
measurement on ferrous substrates uses
magnetic force to quantify coating thickness,
whereas film gauges for non-ferrous metals
use eddy current technology. Some film gauges
have both mechanisms in the same instrument,
others do not. I wonder if perhaps the film
thickness measurement is suspect. What do you
know about the customer’s instrumentation and
If this is not the issue there may be a
difference in path to ground for the aluminum
vs. the steel substrates. Are the same hooks used
for both metals? Are they in different condition?
Poor film build is most often caused by a poor
ground. The electrons emitted by the tip of the
gun have nowhere to go and consequently the
powder does not build.
So, what to do? Check out the instrument
issue and verify the thickness measurements.
Then investigate the continuity to ground. This
is best accomplished with a megohmmeter, also
known as a Megger. The resistance of the part
to the ground must not exceed 1MΩ. If it does,
then investigate all connections from the part
to earth. Ensure that they are clean and making
good contact. Thanks for the question. - Joe Powder
Zeroing in on Pinhole Problem
Q: Dear Mr. Joe,
What are the possible reasons for micro
pinholes in powder coating?
A: Dear Mr. Ramesh,
This is a very common, but an oftentimes
perplexing, problem with powder coatings. And
there are many possible causes for the incidence
of pinholes. Let’s run down the list.
1. Hot dipped galvanized steel (HDG).
This is notorious for causing pinholes
in a powder coating finish. Some say it’s
due to the evolution of hydrogen, others
think it could be from entrapped air/
moisture. Regardless, it is always wise to
preheat galvanized steel prior to applying
a powder coating. You can let it cool to
almost room temperature, but you better
get the powder on quick or the pinholes
2. Volatiles of cure. TGIC-free polyesters
(a.k.a. hydroxy-alkyl amide or Primid™)
will exhibit pinholes if applied too thick.
Films over 100 microns typically show
this defect. Ditto for most polyurethane
powders. They emit a blocking agent,
typically e-caprolactam, which causes
pinholes at thick films.
3. Moisture. If the powder container has
been left open in a humid environment,
the coating may absorb moisture which
can cause pinholes during film formation
in your oven. A moist substrate can cause
the same problem. So, keep your powder dry
and make sure your substrate is completely dry
before applying the powder.
4. Bad or old powder. If your powder is more
than two years old, it may have picked up
moisture even if the container appears to be
sealed. Try fresh powder to see if the problem
5. Contamination. Cross contamination between
dissimilar powders could be the culprit.
Keep your powders separate and thoroughly
clean your application equipment when
switching powders. Defects caused by cross
contamination can range from lower gloss to
pinholes or even craters.
These are the first places I would investigate to
zero in on a pinhole problem. Good luck and let me
know if you have further questions.
Best regards, -Joe Powder
Pinholes like those seen here are a common but perplexing problem
with powder coatings. They can happen for a range of reasons,
including those listed in Joe Powder’s answer above.
Fill in the Gap
Q: Dear Joe Powder,
I need to fill some gaps I have in a welded
aluminum frame and I was wondering what
was the best type of “bondo-type” filler
that wouldn’t outgas too badly and ruin my
finished powder coat. I would prefer an epoxy
or something that wouldn’t necessarily
need a preheat or super long cure time, but
I do understand there are tradeoffs and
I probably won’t be able to find a product
that applies to metal, dries and sands
smooth and doesn’t out gas. Any product
recommendations would be great.
Thanks, Matt K.
A: Hi Matt,
Sorry for the late response; your query
slipped through the cracks. Your observation
regarding fillers is right on the money.
Traditional auto body fillers based on polyester
resin (e.g., Bondo®) work fine for roomtemperature
cure applications such as refinish
paint. They blister and outgas at elevated
temperatures typical for powder coating cure.
There is a solution, however. Alvin Products has
a product called Lab-metal that works well as a
high-temperature filler for parts to be powder
coated. (See http://www.alvinproducts.com/Product-Line/aT/View/ProductID/3/Lab-metal)
It’s a one-part, aluminum filled product that
is easy to apply. Please note that they specify
that Lab-metal is good to 350°F. Hence, the
powder coating you use will have to cure at a
temperature below that. Low-temperature cure
powders are fairly common, so consult your
coating supplier for the powder that’s right for
your application. -Joe Powder
Joe Powder is our technical editor, Kevin Biller. Please
send your questions and comments to Joe Powder at
Editor’s Note: Letters to and responses from Joe Powder have been
edited for space and style.
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