Quote:
Originally Posted by Buggsy2
I see two uses of primer.
One, to prepare the metal or fiberglass for the final top coat of paint. This would be typical of visible surfaces. Sealing, corrosion prevention, etc. are not important for this use because the top coat(s) of paint provide protection.
Second, to protect the metal or fiberglass from the environment, wear, and corrosion. This might be used on interior where appearance doesn't matter. An Alclad surface would offer good corrosion protection, though maybe not for physical wear, and all cut edges would not enjoy the Alclad layer.
Seems to me that a primer that's good for use #1 might not work for use #2, and vice-versa. For instance, somewhere I got the notion that the acid-etch primers are great for preparing the surface for top coats, but don't provide protection against corrosion.
Comments?
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On the exterior surface
filiform corrosion is expected. The top coat, usually polyurethane, is the reason for filiform corrosion. To prevent it a primer is needed.
Traditionally only alodine-primer-top coat would give good protection, but today good primers are formulated without the use of chromates. They give excellent protection against filiform corrosion. The catch is, that is the only thing they are good for (in addition to adhesion of course). It is better to think of primer and top coat as one singe product, used alone they are no good. Following the manufacturers procedures is essential. Typically such a primer needs to be covered by the top coat within hours. Used alone, many primers are porous and will accelerate corrosion because they collect moisture.
Other primers, such as chromated epoxy primers made for aviation applications, are used for internal surfaces. You cannot go wrong with a chromated epoxy primer on the internals. They are strong, durable, and waterproof. When using an "aviation grade" primer, they cover in a very thin layer. The only thing I can think of is it's like shooting small birds with cannons. What Vans are using on the internals on the QBs is probably more adequate for the job, but I don't think it is any easier to apply than an epoxy primer, so why bother?
For the internals, crevice corrosion is the only thing to think about. This corrosion typically occurs between mating surfaces. Chromated epoxy on everything is the brute force method. What is needed is merely a coating between the mating surfaces, but again, it's easier to just cover everything when you're at it, looks much nicer too.
For the internals, it is important that the primer do not create an environment for filiform corrosion. It is important that it is not porous, but repels water. It should be physically and chemically strong. Chromated epoxy is made for this. On the outside, it is much more important that the primer(s) and top coat(s) are applied according to manufacturer's spec.
But then again if you live and fly in a dry climate far from the coast (salt water), store it in a dry hangar, your airplane (or yourself) will become old and useless long before corrosion takes it, even "unprimed". Living and flying near the coast is a totally different matter.
A common misconception about alclad is it doesn't protect at edges and scratches. The corrosion protection mechanism of alclad is
galvanic protection. The pure Al or AlZn alloy of the clad is a sacrificial anode to the AlCu alloy of the core. At edges and scratches the clad will corrode, while the core will not. The clad work exactly like sacrificial Zn anodes used in boats.
I am also looking into rattle cans, because it is faster. So far the only solution I have found is one particular that applies in a very thin layer, and then is covered with a thin transparent top coat to repel water and create
some physical strength. That is a two step process, and seems a bit silly, but it works. But is it faster than using two component aviation grade epoxy? Another one is a rattle can primer for marine application (aluminium boats and engines, propellers etc). That one has pure aluminium pigments, thus it works just like alclad (galvanic protection). However it is a total mess to apply in anything but a thick heavy layer, so it is useless.