Interior surface primer for corrosion control is a hotly debated topic to say the least. Options are to prime everything with 2 part epoxy primer for the ultimate bullet proof finish and protection, or to prime nothing at all. I know people on both sides of the fence. After a lot of thought, I'm going to hit a middle ground and use Stewart Systems' EkoPrime, which is far less toxic than something like Akzo epoxy primer. It's not as durable, but I think for interior surfaces it's completely fine. The huge benefit of EkoPrime is that it is water borne and is not 2 parts, so what you don't spray can just be saved for later. With a 2 part primer, what you don't spray is just waste. The other benefit of EkoPrime is that it can be cleaned up with water. For small parts, I'll just grab some self etching primer in a spray can. No need to drag out the paint gun and mix everything up for little one-off needs. In most climates I would probably take the approach of just spraying mating surfaces and call it a day. Considering I will likely have to have my plane outside (hopefully in a carport style hangar at a minimum) until an enclosed hangar is available, I think it probably makes sense to just prime all interior surfaces. Maybe not in a dry climate like Colorado, but in the Pacific Northwest, the moisture is a problem for most of the year.
I emailed Stewart Systems regarding how to use EkoEtch and EkoPrime:
We recommend etching the aluminum by spraying it lightly with EkoEtch mixed with water and then rubbing it for a few minutes with Scotch-Brite. Then rinse with water. It is critical to do this on any surface that will be painted. On interior parts if you were doing primer only corrosion protection you could just clean it with eco-clean and spray it with EkoPrime. It will not get as good of adhesion, but on an internal surface that will not be disturbed it should not be an issue. It will provide good corrosion protection. In both cases we recommend cleaning with EkoClean first.
Good to know I don't have to go crazy with etching for internal surfaces. That being said, I think I will still probably go the route of EkoClean, followed by EkoEtch (scuffing with Scotch Brite) and then primer. With the way riveting can beat things up, I think having the best adhesion possible is probably a good idea. Depending on how EkoPrime looks and works for closed areas will determine if I use it for the cockpit or if I go with something more durable. I will probably cover most of the cockpit with panels to finish it off, so don't have a burning desire to paint inside if a primer looks okay and is hard enough. Stewart also makes a water borne epoxy primer, which could be a solution for a higher touch area like the cockpit. I'll cross that bridge down the road.
I did just find a used Devilbiss Finishline HVLP paint gun for cheap, so one more tool covered. I think I will get the Devilbiss DeKups system too, just to make cleaning up and saving primer a little easier. They also enable you to spray at odd angles, whereas that can be a little tougher with a normal gravity fed gun setup.
