Do yourself a favor, as I posted earlier in this thread, before painting the inside of the tubes, convert the rust to ferrous phosphate by applying a weak solution of phosphoric acid (this is sold at Home Depot in the paint section and is commonly called metal etch or prep, etc.) Sold on Amazon too.
At Amazon
You can spray it, brush it, swab it on, then wash it out with water after the time period (typically 15 to 30 minutes) in the directions. Let dry and then paint. This provides a good base for the paint to adhere to and you won't be trying to paint over rust - which seldom works well.