I hosed down the inside of every tube on my recent build with T9 Boeshield. If i had Framesaver i would've used that, but i'm cheap and used what i had on hand. It's well known as a rust inhibitor so i'm not too worried.

Spray, flip frame over afew times, repeat as necessary until you have it dripping out of every vent and tube. Hang frame, let dry/drain overnight, wipe down bottom bracket shell threading (it's going to get greased), shake out excess if possible, and assemble bike.