I have been trying to find a suitable solution to the priming issue that is safe, effective and consumes minimal time. I believe I may have a reasonable compromise, provided I'm not missing something (there's a few things I am unsure about, see below). I'd be interested to hear your thoughts. First, a high level summary of the available options used to date.
1) Prime with a chromate based primer (+/- Alodine)
Pros: Most effective anti-corrosion option.
Cons: Time consuming, hard work, toxic. Removes protective alclad layer (if scotch brite used rather than alodine). Expensive.
2) Prime with rattle can primer.
Pros: Convenient. May work.
Cons: Still a fair bit of work and somewhat toxic. Removes protective alclad layer (if used with scotch brite). Unless using chromate-based product, it offers minimal cathodic protection and relies on providing a weatherproof barrier of varying integrity and adhesion. Expensive. Susceptible to damage while constructing.
3) Prime with 2-pack, non-chromate primer
Pros: Less toxic than chromate options. Probably reasonably effective.
Cons: Lots of work, pretty toxic, inconvenient.
4) Don't prime, treat with Corrosion-X or similar.
Pros: Very convenient. Preserves protective alclad. Plane gets built in much less time!
Cons: Messy for years afterwards (apparently), relies on accessing all parts of the interior airframe with a spray. Not 100% familiar with the product but understand it remains a liquid.
So, what if there was something that provided a solid robust barrier like epoxy, was as convenient as Corrosion-X, non-toxic and didn't remove the alclad? I think there might be.
I want to present a product I have used in automotive restorations for years - Penetrol. If you have a sill that is rusting out, this is the best way to stop it in its tracks. It's like a 21st century version of fish-oil, but stops smelling, sets hard and is more robust.
You can find info about this product here:
This product starts out as a relatively low toxicity, super thin oil that can penetrate anything like WD40 spray. Seriously, it finds its way into every crevice. It will easily work its way under ribs and skins, if used as I outline below. Then, after application, it slowly hardens into a very tough clear flexible film and stays this way up to 100 degC (very hot). It's so flexible that the manufacturer has videos of it sticking to plastic bags. However, it is also easily as hard and adhesive as rattle can primer, probably more. So we have something like corrosion-X, which after being applied goes hard and is reasonably permanent. So it won't keep running out of the cracks for years afterwards and spoiling your paint. You have the protection of a basic primer, without removing the alclad, and almost no preparation. It will also fill tight crevices and prevent water from working its way in. One thing I am unsure about is how well it will stick to Aluminium, but I think it will be ok (going to do a test soon).
I imagine a treatment workflow going like this.
1) Clean parts with acetone or alcohol to remove oil.
2) Completely assemble component, e.g. rudder.
3) Treat insides with excessive amounts of Penetrol using spray, rotating part in all orientations to ensure spray gets into all crevices and onto all surfaces. Collect run-off in container to use another time.
4) Hang part up length ways with bucket to collect drips.
5) Once in a while over the next few days, rotate part to prevent pooling in any internal location, remove any penetrol that finds its way onto the outside using a rag. It is important to rotate regularly at the start while the oil is still thin enough to work its way out.
6) Leave for extended period to harden once dripping has stopped.
Of course this assumes it will stick to Aluminium. If not, I am sure using phosphoric acid to etch it would be a suitable treatment. The Manufacturer does say it is ok on non-ferrous metals.
Also, some parts could be lightly coated while assembling if desired. It comes in spray cans and 1L tins.
In line with above, let's discuss the pros and cons:
5) Treat with Penetrol
Pros: Super easy, should be effective in providing permanent weatherproof barrier to prevent corrosion. Preserves Alclad. Low toxicity.
Cons: Flammability - does it matter? Weight - should be ok but unsure, will depend partly on how carefully excess is removed while it's wet. No heavy pigments at least. No cathodic protection. Will it stick to Aluminium ok?
I reckon this treatment might offer a great compromise to the priming issue, unless some of the factors I outline in the cons section come in to play. Definitely not sure how thick and heavy it will end up being, but hopefully less than epoxy??
Keen to hear your thoughts, I'm sure someone has come up with similar things before.