I built a floating dock out of old pontoons. My BIL had purchased a set of 24', steel units from the scrapyard. He had them torched in half so he could transport em', then kept the "bow" sections for himself, for a future mini-houseboat project.
I purchased the "stern" sections off of him to use for my dock. I got lucky, as when the salvage yard workers cut them in half, it left an internal bulkhead in the rear sections, where as he had wide open tubes to try and plug up. I just welded a bead around the bulkhead to seal em' up.
I welded a bung into the top of each half, and installed screw in Schrader valves. I kept the existing drain plugs in the bottoms.
It took a long day of welding seams and holes, then pressurizing, then checking for leaks with soapy water, then de-pressurizing so I could weld up the leak. Repeat scenario umpteen times.
When I was satisfied they were as watertight as I could make them, I painted the pontoons with rubberized , waterproof mastic. I figured the black coloration would cause the air trapped inside to expand, helping to prevent water entry. When you depress the schrader valves on a sunny day, air rushes out.
BUT... about once a season, (I pull the dock out in the winter), I end up carrying a portable air tank over and adding pressure to one of the pontoons, then opening the drain to force the water out. At this point the leak has not worsened, so I am content just to keep an eye on it.
I checked on filling the pontoons with foam. VERY expensive.
