Don't forget - with all the inherent "leakability" any treament needs to let water run through. I've heard horror stories of people blocking drains and ending up with rust troubles higher up the structure. I personally like the idea of sticky oil rather than the idea of a thick creamy wax but as you say you do need to get things as high up as possible.Hi. Good to hear from you.
Good thinking about some extra protection. I quite fancy injecting the protection with it upside down so it runs to the top the with it standing on either end so it gloops all around the inside. So many water traps in there it's crazy.
"Industrial" solutions often use high pressure air and nozzles to make an atomising spray (but seriously you need to have finshed your paint work and let it dry before you do this kind of treatment)
If I'd have logged in sooner I would have suggested using something like Zinga on the inside of the cavities whilst they were still open. You can weld through it but it is really dodgy to do so in an area with out decent ventilation. Zinga is essentially liquid zinc (96% Zinc - probably measured by weight - apparently) which I have been using quite a bit recently. I like it for inside structures (and the steel frame I've built for my new shed) but as I say it is a bit dodgy for welding so you need to be sensible / aware / reckless / irresponsible / not caring about life...