General Notes on Corrosion Protection
Primer Wars!
Corrosion protection is a
highly volatile topic in the RV community, sometimes referred to as the
"primer wars". There's a fair amount of information out
there, as well as a fair amount of misinformation. The often debated
factors include necessity, effectiveness, weight, cost, time, as well as
personal health and environmental risks associated with various corrosion
protection products and methods.
Empennage
After doing my own research
and analysis, I decided to implement one of the more robust solutions
available when building the empennage. The approach is to treat all
internal components, including the interior and/or overlapping surfaces of
skins, with an alodine chemical conversion coating followed by a
purpose-designed sealing epoxy primer. This solution entails a
significant investment of time and money, plus a slight weight penalty,
but is highly effective. I chose this solution for the empennage
because it will be difficult to treat or even inspect inside the tail
surfaces for corrosion in the completed airplane. Also, I simply
wanted to gain experience with this process. Especially with a spray
gun, which is used for applying the epoxy primer.
Alodine
Alodine must be applied to a
clean, corrosion free surface. My surface preparation begins
with a thorough wipe down using acetone and paper towels. This
removes most ink, grease, oils, finger prints, and other
contaminants. Then abrade with medium (maroon) or fine (gray)
scotchbrite pads while etching with alumiprep 33 (phosphoric acid) diluted about 3:1 to
remove existing surface oxidation. Rinse with water and check for
even sheeting (no beading), indicating a successfully cleaned
surface. Dry with a compressed air nozzle or paper towels.
Thus far I've used two
varieties of alodine and several methods of application. Alodine
1201 comes by the gallon in plastic bottles (about $15/gal from Aircraft
Spruce), and the preferred method of application is to immerse in it for 3 minutes,
then rinse in water. When full immersion is not practical, brushing
it on also works ok. But the alodine 1201 should never be allowed to
dry. After 3 minutes, rinse it off. Alodine 1201 is not so
environmentally friendly in that some of it inevitably ends up in waste
water. Also, just handling large volumes of this carcinogenic liquid
is a bit disconcerting, even with proper precautions (overalls, gloves,
respirator, splash goggles). An alternative product I've discovered
recently is the "Touch-N-Prep" alodine 1132 pen from Henkel
Surface Technologies ($38.50 from SteinAir). This product dispenses
a thin film of alodine like a big magic marker. And unlike 1201,
this form of alodine is simply allowed to dry on the surface. There
is no rinsing, making it less messy and more environmentally
friendly. The manufacturer recommends two coats at 90 degrees to
each other.
Interior Epoxy Primer
I chose to use an epoxy primer
from Akzo-Nobel. More on this later...
References
