Some framebuilders use linseed oil. Its cheap and available at Home Depot in the paint section. A gallon costs next to nothing. Boil some in a pot and pour it into each tube. Make sure all the inner tubes are coated especially the rust prone chainstays. Turn the bike upside down, sideways, righside up, and every which way to get the linseed oil to drain out. Let it sit outside for a few hours to dry. Remember to wipe off the excess. Presto! Rust protection that will last for years!