Let me add that the primer Robinson uses is utter crap. Yes its environmentally friendly for California but in real life down on the gulf coast of the United States their aircraft corrode and fast. We've seen an aircraft with 147 hours on the clock and in for its second annual exhibit corrosion on the trailing edge of the blades, around fasteners, etc. While doing the blade service bulletin that lets you file off the edge We were told by Robinson that we could use epoxy polymide primer instead of the stuff they send in the kit. They by law cannot use certain primers for environmental concerns.
Now we are talking about the real issue in adhesive bonding, and the same technology applies to corrosion and paint management. As I have posted many times, adhesive bonds rely on chemical reactions at the interface between the adhesive and the substrate, and the basic principles apply to paint adhesion as well. The common perception in adhesives and paint technology is that the surface must be clean. However, California has banned most of the reliable solvents because of VOC regulations. It is possible using low VOC solvents but the risk is that the slow evaporation rates may result in spreading any contamination by deposition of residual contaminants.
The requirements for a clean surface are true but only part of the story. To enable the chemical bonds to occur, the surface must also be chemically active to enable the reactions to occur. SO IT IS NOT SUFFICIENT TO JUST HAVE A CLEAN SURFACE.
For metals, you need some treatment to remove existing oxides so that the surface can be chemically active. This can be performed by etching (but these processes usually involve the nasty chemicals to be avoided in California). But is may also be performed using mechanical means such as abrasion by particles or by hand abrasion with abrasive papers etc., but hand abrasion is known to be the least reliable process. So it is possible to do this process and still meet CA regulations.
Now I stress that if the bond is performed with cleaning and oxide removal ONLY, then the initial strength if tested will convince you that the process gives a good bond strength.
This is the crux of the matter, because the regulations require demonstration of static strength, damage tolerance and fatigue resistance and if these tests are conducted soon after the bond is formed.
There is no requirement to demonstrate that the strength, damage tolerance and fatigue resistance are sustained throughout the life of the bond.
What is missing is that the primary cause of adhesive bond failures in service is hydration of the oxide surface formed at the time the bond is performed. The chemical bonds at the time of bonding must dissociate to enable the oxides to hydrate, and in the process the adhesive disbonds from the adherend.
The same is true for why paint flakes off surfaces.
Now it is possible to prevent hydration of oxides on metal surfaces by simple treatments with coupling agents which form strong covalent bonds to the fresh oxides, and it takes more chemical energy to dissociate the coupling agent bonds than is required for hydration, so hydration does not occur. The coupling agents are polymers and while one end of the polymer chain reacts with the oxides, the other end must be selected to react with the material at the other side of the bond (adhesive or paint).
Now what does this imply? If I treat a surface to provide a clean, chemically active surface and then treat that surface with an appropriate coupling agent that prevents hydration and is selected to match the adhesive or paint, then hydration will not occur, so disbonding and paint failure will not occur.
Now it is also important to understand that corrosion is nothing more than advanced hydration of the surface of metals, so if a metallic surface is clean, chemically active and treated with an appropriate coupling agent not only will the adhesive stay bonded, paint will also stay bonded and corrosion should be minimised if not prevented. If you understand this, then you will also understand that the idea that corrosion is the cause of disbonds is bovine excrement.
It is also important to understand that the application of a primer on a metal surface unless it is clean and chemically activated, and is compatible with the materials being applied you are wasting your time.
Regards
Blakmax