How thick of metal for frame connectors

Anyone know what is an easily sprayable (thin) product for rust conversion/prevention for this purpose, and other cavities? Like the rockers. I cut holes in mine front, back, and one middle (inside where it won't be seen) to get sand out after blasting. This and many other cavities should get some treatment.

I'd like to do something like the Eastwood system without paying their premium for what no doubt is a simple product with fancy marketing. It would take quite a few of their spendy aerosols to do everything. I'm thinking a garden sprayer with a wide angle tip might be useful.