Kind of wondering out loud here - I notice that Boeing apparently uses primer inside of their aircraft structure. Most airliners are scrapped in 20 years or so - so why does Boeing use primer inside? And what do they use? Does anyone have this knowledge - I'm interested to learn the answers.
Ooooh goodie, a chance to reignite the primer wars.
The short answer is that Boeing prime their aircraft because they are professional, they are smart, they have enormous experience, and they know that unprotected or alclad aluminium in certain enviroments can start corroding almost immediately.
The VansAirforce archives are full of reports of RV builders developing corrosion on bare alclad components....during the build process!!! They come up all the time. So much for unprotected Alclad. Here's a thread posted just a couple of weeks ago:
http://www.vansairforce.com/community/showthread.php?t=79813&highlight=corrosion
It should tell you something when even Scrooge McVan uses a primer on his quickbuilds (not much of a primer mind you).
On aluminium components Boeing actually uses a Boeing Spec product, namely BMS 10-11 Type 1, which is a two-pack epoxy primer and typically comes in a green or yellow colour.
In reality virtually all of the reputable general aviation manufacturers now use the same BMS 10-11 spec product including Piper and Cessna.
It is relevant that decades ago many major GA aircraft manufacturers, including Piper and Cessna, never used any primer at all. The end result has been an international fleet of GA aircraft with massive corrosion problems which have left today's owners with a corrosion repair bill that totals literally hundreds of millions of dollars annually.
You'll hear anecdotal reports from some owner living in the Mojave Desert about how good his non-primed alclad-only Cessna from 1975 looks...but the truth is that a large percentage of the ageing GA fleet are now virtual rust buckets. You only have to pop a piece of alclad aluminium in a 5% saltspray cabinet for a mere couple of days to realise that alclad (a thin, pure aluminium coating) alone provides
extremely limited corrosion protection. It's better than nothing....but not by much.
You may ask yourself why so many RV builders do not properly prime their aircraft given the experience of major GA players such as Piper and Cessna. The reasons are many but I will list a few.
1. Ignorance of the limited protective capabilities of Alclad.
2. Desire to save money.
3. Desire to save construction time.
4. Building for their own limited flying future rather than the possible extended flying future of the aircraft.
5. Encouragement from other builders who are influenced by 1-4 and feel the need to rationalise their "easy option" building technique.