Corrosion inhibitor
I want to second this:
We all don't live in dry environments. I’m a big proponent of fogging airframes with preservative. There are some very good products out there. It’s some of the cheapest insurance anyone can purchase for their investment. A flying, airworthy aircraft has a lot more value than a bunch of spar parts that formerly flew in close formation.
While prime, and optionally paint, is most often provided as the go-to corrosion prevention, as others pointed out, it prevents visual inspection of the component.
Also, if any imperfections in the application occurred, or the primer is scratched during or after installation, corrosion may occur hidden from view. Another alternative is provide a sacrificial anode for the ongoing galvanic activity that is occurring.
How many of you, myself included, have primed dissimilar metals in preparation for riveting together, and then while assembling realized that an alignment tool was needed, or that one hole that needed reaming, or the rivet was a little snug - so you just pushed hard and shoved that little sucker in there? In all of those cases, you very likely abraded the primer coat and bare metal now exists within the structure, hidden from view, making a premium location for corrosion to occur. This is where inhibitors come in that can find there way into the most minute gaps around rivets, between close fitting parts, etc...
My preference is CorrosionX and then just spray out the entire airframe, (fine mist...don't overdo it) every other year (maybe annually if in a high moisture/salt air environment), however, there are several other corrosion inhibitors in the market, such LPS-3, or CRC's Heavy Duty Corrosion Inhibitor that are easy to apply with either aerosol can or brush. The best option is to apply regularly
after painting, but can be applied to ongoing corrosion areas to react with and inhibit further corrosion.