Today I took apart all components of my Centurion Pro Tour project and noticed there is some rust coming from the downtube mainly (observed from the bottom bracket hole)
The Downtube has bosses for water cage above and below the tube. When I got the bike, I only had bolts on the bottom holes.

What is the best way to clean the rust from inside the tube before applying some protective oil?
I have evapo rust home and used it before with success, but I have to leave soaked for a while.