how to rustproof empty cavities?
 

I have a project car that I suspect has surface rust inside the rockers / longs. I would like to spray something like Ospho or other rust treatment in there but how? I know I can drill holes that can be covered by rocker covers but how do I spray this stuff? I'd like to find a canister that I can fill with Ospho and put air in it with a thin hose/tube to get into tight areas. Any suggestions?

Harbor Freight sells an item called a solvent/cleaner spray gun,cost $20.00. I used it to do the job you described. First I did what could be reached with the nozzle the gun came with.
Then I drilled the nozzle out to push a 1/4" id copper tube into it and got the areas way back in there. Read instrurtions well on Ospho, I think surfaces treated with it have to be throughly flushed with water before painting. I used a rust converter that when cured also acted as a primer ,then I sprayed ,using the same spray gun Rustolem rusty metal primer
until it ran out of every opening. my car was on a rotisorary so it was easy to spread the paint inside.Hope This Helps.