Rust prevention: Inside fuselage

Elsewhere I"m listening to a discussion about using boiled linseed
oil/Tubeseal to protect against rust on the inside of the fuselage.
There appear to be two views: one is start pouring the oil in thru a
small hole until full, then seal the hole. The second view is fill her
up, slosh around to coat, drain and leave the hole open.

Is there an authoratative source for which approach to take?

In either case (unless the fuselage was specifically built to be a
seaplane), tubing isn't interconnected (correct me if I'm wrong), so
if you you drill a hole in the lower left longeron and pour in oil,
all that will be treated is the lower left longeon - not the tailpost,
not cross tubing. Correct?

- Mike