Hello Troy,
The idea is good, but butyl tape can dry and shrink over time.
I used a roofing product that has aluminum on one side and a rubbery tar on the other, with the sticky side down.
It keeps water from collecting between the wood and steel frame, and helps to keep the paint on the frame from rubbing off.
The bottom of the plywood floor was painted with several coats of oil based paint.
Whatever you use should go all the way across the frame members, as just a strip of something down the middle of a rail or support can leave a place for mud and water to collect.
Rob
