The problem with painting aluminum is getting the paint to stick. Boeing
has several (expired) patents for anodize processes that make adhesives (or
paint) stick to aluminum. The process also works for electroplating
aluminum (which is where I have used the process). Get your parts
phosphoric acid anodized (not the more common sulfuric acid process) and
tell the anodizing shop to skip the step for sealing the anodize. Retrieve
the parts as soon as they are anodized and apply the primer coat immediately
(as soon as possible) to get best adhesion; the finish coats can wait.
There is as usual a trade off: seal the anodize for maximum corrosion
resistance or don't seal it for the best adhesion of the paint. It's like
Rodney Dangerfield's question: married or single, what kind of choice is
that?
Anodizing per MIL-A-8625 is required prior to painting US military aircraft.
For complete details consult the Metal Finishing magazine's 2000 Organic
Finishing Guidebook & Directory, page 81.
Following solution (heat) treatment, aluminum alloys are artificially aged
to T6 or naturally aged to T4. It has been a very long time since I was a
process engineer for an aluminum mill but as I recall the artificial age
cycles were on the order of 8 to 16 hours at 350 to 450F depending on the
alloy. It is possible to "overage" the material but I doubt that there is
enough soak time in the powder coating process to have a detrimental effect.
Be thankful for those winds. Here (SoCal) the winds are so boringly
consistent that I was only a few weeks away from my checkride before I got
any real experience in crosswind landings. Now I know that even a novice
can land a C-152 with a crosswind component that exceeds the "demonstrated"
component. It's a good confidence builder. And while we would all agree
that the C-152 is slow (jokes about bird strikes from the rear are somewhat
of an exaggeration), what it really doesn't do well is climb. One of my
cross country trips with my CFI was the just over 60NM trip from SNA (54
MSL) to L35 (6748 MSL, but in a valley) and we had to circle for a long time
to gain enough altitude to get over the mountain that separates the LA basin
---From Big Bear even though we were climbing constantly at Vy following
departure. The service ceiling in the POH (13,500, I think) seems somewhat
optimistic.
Best regards,
Rob Housman
A070
-----Original Message-----
Behalf Of EuropaChris@netscape.net
Subject: Painting of aluminum bits and pieces
Hopefully I'm not rehashing old news here:
We are close to the final assembly of our wings, and there is a lot of parts
that are bare aluminum (the OR hardware, etc.). We've been doing the
Alodine treatment to all internal aluminum parts before bonding, but haven't
given a lot of thought to external hardware. We also have some zinc
chromate primer available.
My question is twofold: (1) is it better to have the individual bits painted
(either powder coat and/or prime and spray paint), and then assemble the
wing mechanism with the raw hardware, or better to Alodine the alu bits and
spray the hole works when the wing is painted, hardware and all? If the
answer is (1), does anyone have a list of all the hardware items (wing and
fuse) that they powder coated so I can riffle through our inventory to get
all the parts together for a single paint run?
The other thought I have is what does the 400 deg. F temps in the powder
coating oven do to the heat treatment of the aluminum? From my materials
classes in college, I seem to remember the temp. to age aluminum is around
this number. It seems to me that after a thorough baking at 400, the
properties of the aluminum would be changed. For non-structural stuff, it
won't matter much, but for an airplane.....just wondering.
Thanks for the help!
Chris Beck
A159
PS Hopefully soon I'll have the opportunity to get some time in the 152
where I do not have to fight 20kt. winds. Makes for interesting landings
while training. Wisconsin sure is windy in the spring - and 152's sure are
slow.
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