Re: Protective coatings - Pros vs. Cons
Posted: Sun Jun 09, 2013 1:00 pm
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.
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.
