by Ron Joseph
Selection of Spray Guns--Transfer Efficiency
Q. Having read your trials between AAA and Airless paint sprayers I find that
the Airless was given the short end of the staff if you know what I mean. Taking
a setup such as AAA which is pretty much setup for wood finishes vs. the airless
which is setup for house painters off the showroom floor. My question is this:
Do you honestly think that an air atomized system is more efficient than applying
a coating without air atomization? Also, How can you be nuetral in your bias
when you are funded (website) by the E.P.A.? Thank you for your reply in advance.
First let me assure you that I am not funded by anyone. I'm an independent
consultant who answers questions for free. I get nothing either from the EPA
or the equipment vendors. I am not employed by the folks who operate the website,
nor do I have any axes to grind. When I answer questions I look solely at science
and technology and leave out politics.
Now to your question. I don't remember the question that you are referring
to since I answer at least 4-5 questions every day. For industrial finishing,
such as sheet metal, plastic parts, automotive, etc., air atomization is essential
for breaking up paint articles small enough to produce a high quality, smooth
glossy finishes. Therefore, you will almost never see Air-Assisted Airless (AAA)
or Airless being used in an automotive finishing shop for decorative finishes.
I have visited GM, Toyota, Subaru and Honda facilities and have only seen conventional
and HVLP spray guns being used. (Here I'm excluding electrostatic bells and
disks, since your question relates solely to manual hand application.) Similarly,
I have been to over 40 aerospace facilities and numerous high production factories
where one of these two air atomizing guns have been used.
In wood finishing AAA is commonly used, but wood substrates are different from
hard impenetrable metal substrates, and while AAA and Airless can deposit large
volumes of paint quickly, it is difficult for a painter to get a Class A finish
with either gun.
Air assisted airless and airless guns do have a place in our industry and are
used extensively where painters need to apply large volumes quickly. Also, these
guns have the advantage of being able to atomize high viscosity coatings, some
of which simply can't be adequately atomized by air atomization. I have seen
AAA being used to paint large metal cabinets and electrical switch boxes, but
this is unusual.
In the ship building industry airless is the most popular spray gun because
it applies huge volumes of paint to the hull of a ship at very high film builds,
up to 10 mils and more in a single application. Airless is also the most commonly
used technology for architectural applications, but here we are not looking
for Class A finishes. If architectural paints weren't formulated with thixotropic
properties, the paints would easily run and sag, but acrylic latex's are wonderful
for such applications because of their thixotropy.
Let me address the EPA and regulations. In some of the EPA's surface coating
regulations the use of airless and air-assisted airless are prohibited, solely
because the EPA believed (back in the mid-1980s) that these spray guns yielded
poorer transfer efficiency than HVLP and electrostatic guns. I disagreed then
as I do now with some of their rationale, especially with the banning of AAA
guns for certain applications. But I don't work for the EPA nor do I write their
regulations. Back in the 1980s when I performed transfer efficiency tests using
a wide range of spray guns the regulatory agencies especially didn't want AAA
or Airless to be used to paint small parts because they asserted, correctly,
that for most such applications transfer efficiency would be unacceptably low,
when compared with HVLP. I had extensive discussions with the regulatory folks
at the Bay Area Air Quality Management District (BAAQMD) and the South Coast
Air Quality Management District (SCAQMD), who were the leaders in regulating
spray gun applications. They agreed that airless and AAA could be transfer efficient
for painting large parts, but they were concerned that from an enforcement perspective
inspectors would not be able to distinguish a "small" part from a
"medium" part. We tired to establish definitions for each, but found
it to be impossible. Other considerations, such as the complexity of the part
geometry played into this, and in the end the two regulatory agencies, (BAAQMD
and SCAQMD) decided to simply disallow AA and airless for the painting of Miscellaneous
Metal Parts and Products, and many of their related regulations. The EPA picked
up on this and in the Aerospace NESHAP, which was published in 1995 essentially
followed California's example. (Ironically, I do think that AAA can be more
transfer efficient for the painting of aircraft, but the Rule developer at the
EPA disagreed.) To the best of my knowledge neither the EPA nor the state agencies
have ever banned the use of AAA and Airless spray guns for ship building, architectural
and similar applications, because it has been clear from the start that these
two spray guns are ideal for such industries.
(This answer will be published in the November issue of Metal Finishing Magazine.
There will be no reference to you or your company and the Q&A will be generic.
Moreover, I will reword your question.)