Protective coatings - Pros vs. Cons

[psalter]

You are never going to find a definitive answer on this subject.

As an engineer for the US Navy. We alodine, prime and paint every part before assembly. We used to use zinc chromate for the primer, which is the best for aluminum, but where forced to use epoxy primer due to environmental/health issues. But our aircraft get dosed in salt water/salt fog continuously at sea.

Some corrosion starts from the inside out, and basically no coating is going to stop that.

Epoxy primer is better for steel than zinc chromate. Steel must to be coated, and the prototype Panther steel items were powder coated. Powder coating is more durable than paint.

There are various grades of stainless steel, and contrary to some belief, some of them will rust and are magnetic because of the iron content. Others are extremely durable to elements and non-magnetic.

On the other hand my 1947 Ercoupe was not primed or painted inside, and I had very little corrosion between joints.

As you said, the environment and keeping it dry is the most important thing. Cessna did not prime or paint interior surfaces except for ones on floats until recently. Piper always did.

All corrosion requires moisture. The spray on corrosion preventives are temporary measures at best, and are designed to seep into the joints and displace the moisture, and then keep it out. But they have to be reapplied and does not penetrate tiny spaces. The Navy has found there effectiveness limited, in some areas they do well, others not so well. The navy continuously tries different corrosion treatments but, after 100 years of going to sea with airplanes, no one has found the perfect treatment.

Corrosion always wins in the end, just a question of how long it takes. Moisture takes out wood, metal and composites.

[Tony Spicer]

Ron,

The primer debate is never-ending. Rather than start yet another one, Go here

http://groups.yahoo.com/group/sonextalk/messages

and put primer in the search window. At last count, there were 635 hits.

My Sonex was 12 miles from the ocean, built with all-stainless rivets, and no paint inside or outside. Five years and no problems. For the most part, the prototype is built with Cherry BSC rivets. Aircraft Spruce has all the details on the rivets. Aluminum body with a steel stem.

Tony

[RonFranck]

This is probably a topic that has been debated to death on other builder sites, but I'll ask here because this will be my first aluminum airplane and I have not followed anything elsewhere.

What are the thoughts on protective coatings for your build?

I'm guessing it depends a great deal on the environment where you plan to operate you airplane. Those living in salty, humid air might give more serious consideration to some form of protective coatings while those operating in dry, arid regions would deem it unnecessary. Other factors might include such things as resale value, how long you plan on operating your airframe, added weight (in the case of epoxy primers), time vs. necessity, etc..

If you choose to incorporate some form of protective coating what are you considering? Zinc Chromate, Alodine, Epoxy Primer?

Ron Franck