I think the garden sprayer idea with an anti corrosion spray like Gibbs, Corrosion X, LPS 3 or such should work real well. I plan on painting inside with a rust encapsulator and then spraying down with the above mentioned.
A friend I work with at the helicopter repair station (both of us long term aviaton) said some manufacturers use linseed oil sprayed inside of box and round steel tube when used in aircraft for a corrosion inhibitor.It might be worth a try. Lindseed oil is not anywhere near 100 a gallon like some of the other "name" stuff out there.
Just some of my ideas for ya.
