When I did my frame, I had it chemically dipped. After painting the outside, I used an inexpensive rustproofing spray gun I purchased from J.C. Whitney (also available from Eastwood) with some 3M (I believe) rustroofing to spray inside the frame (heavy dark liquid that semi-dries to a firm rubbery consistency, similar to what Ziebart and other rustproofing processes use) . I taped off all the holes except the one I was using for access at any given time, inserting the wands and spray nozzles that came with the gun. I could gauge the effective coverage of my coating pretty well by the coverage on the pieces of tape I removed from the various holes. Can't tell you how long it lasts, but my guess is that it should be very effective for many years.