[TowerTalk] 2" OD Long Masts

[Byron Tatum]

Hello-
  For years I have used the 2" OD, 6061-T6, 1/4" wall x 24' long aluminum masts to stack VHF and UHF antennas. I have seen these take a bend a time or two over the decades.
  I wanted to ask:
    1. If I was to switch to 2" OD steel, as a mast to extend about 15'-18' out the tower top for stacking VHF/UHF long boom yagis, what would be a suitable grade of steel, the finish and wall thickness?
    2. I have heard of inserting a smaller aluminum tube inside the 2" aluminum mast, in the area where the mast exits the tower top and on above a bit. I wanted to ask if someone here has experience with inserting an aluminum tube inside the 2" OD mast? I would assume it would be 1-1/2" OD, of similar material and length and what wall thickness?
  I guess reasons I have stayed with the aluminum masts is that they are lightweight and easy to handle, plus no rusting down in this Gulf Coast area.
     Thanks, Byron WA5THJ