[Joe Pass]

I had to do some thing similar recently with 4" squares tube 5ft in length. What I did was buy the Rustoleum commercial grade primer and thin it 50 percent. With gloves and plastic on the floor I set up the length's of tubing with one end sealed with tape and proceeded to fill one tube up and from that I poured it into the next one until I did all 5.
I laid them out on saw horses to dry and removed the tape and wiped of the mess on the outside. I finish the outside later with the same primer and painted it with color after that. It was a little messy if you don't prepare everything right in the beginning but the inside of the tubing looked great