Ask Joe Powder
Posted on Wednesday, October 1, 2014
Do You Know Your Type?
Dear Mr. Powder,
What do you think is the best way
to degrease steel and aluminum prior
to coating? I have seen various
options out there, but there seem so
many to choose from I was wondering
what you thought was the best option?
Specifically, when sand blasting
is not an option due to the “coarse”
surface area left. I know sand blasting
can be fine, but it is still difficult
to get a smooth surface.
Thanks in advance, -Jeff
Here is the lowdown on preparing metal for
First, the durability requirements of the
finish must be carefully considered. If this is a
basic indoor application that won’t see much
wear and tear or moisture, heat, etc., then
simple solvent cleaning and possibly media
blasting will suffice. For anything that will reside
outdoors you need more than that.
Powder chemistry has a large effect on durability
and performance. For instance, epoxies and hybrids
adhere well over blasted metal and can provide decent
corrosion resistance with a minimum of surface
preparation. Essentially all other chemistries require
metal cleaning followed by chemical pretreatment.
So if you are using a polyester, acrylic or polyurethane
powder it’s best to clean the metal first with an
alkaline solution, rinse well, then apply a chemical
pretreatment. These are typically phosphate-based
for steel and chromate-based for aluminum. Chromate
is rather hazardous (actually carcinogenic) so
I would strongly recommend that you investigate
alternate materials if you need to pretreat aluminum.
Newer chemical treatment types are emerging
that are based on zirconates, titanates and silanes.
These are essentially non-toxic. Process control with
these is critical and therefore it is necessary to carefully
follow the supplier’s application parameters.
It is also very important to recognize that although
cleaning techniques may be universally
acceptable for both steel and aluminum, chemical
pretreatments do not translate across these very different
metals. Iron phosphate works well on ferrous
substrates; however, it does not prepare aluminum
adequately for high-performance powder coating
performance. You will need chemistry specific to
your metal type.
Mirror Mirror on the Wall,
Who’s the Glossiest of All?
The thought below from some of
my colleagues is close but not perfect.
Anything you can share either from
an ASTM specification or what the
BIG companies use to follow specific
protocol for DOI (distinctness of image)
would be greatly appreciated.
“I conclude that either DOI or gloss
the issue is the smoothness of the
dried top coat. Which in turn depends
on size and shape (of the filler particle)
and how much resin is absorbed
by each particle.”
Thanks, -Michael W.
Distinctness of image is more than just a
gloss measurement. It refers to the crispness
or distinctness of a reflected image. Initially
this was assessed visually by projecting a
series of different sized images onto a coated
surface. This was accomplished by using a
“glow box” that works by passing focused
light through a slide containing these images
of varying sizes. Kind of like a projected eye
chart. See: (see: www.gardco.com/pages/gloss/doimeter.cfm). A little primitive, but it gets the
DOI measurement has since evolved with an
Instrumental technique. This method is described
in ASTM D-5767. (see: www.astm.org/Standards/
D5767.htm and www.elcometer.com/en/laboratory/appearance/distinctiveness-of-image/productmanager?prod=742). Instrumental assessment eliminates
operator influence and is therefore more
precise and reproducible.
DOI can be affected by both long (i.e., orange
peel) and short (i.e., low gloss) wavelength surface
texture. It is important to know how DOI is being
measured and what the target is. Both low gloss
and orange peel will reduce DOI. Hence large particles
of filler in a powder formula that create low
gloss will reduce DOI and also small filler particles
with high oil absorption will reduce DOI.
I hope that this helps.
Joe Powder is our technical editor, Kevin Biller. Please send your questions and comments to Joe Powder at firstname.lastname@example.org