I have run across many older aluminum boats, most are at least 40 years old or more, that used some sort of packing or caulking at the rear around the edge of the transom. It looks almost like lead or plumbing packing. The first one I saw with it was an early 60's row boat. (Since NJ doesn't title boats under 12', I often have no idea of the original manufacturer or year). I had one the other day that I was told was an old MFG or Gamefisher, the hull construction was of three main pieces, a left and right side, and the transom panel that was riveted inside with three 90 degree angle brackets all around. The resulting seam was packed and smoothed with what looked like a silver or lead packing, it wasn't real hard and I was able to chip it away with a screwdriver. The boat had about 10 coats of old paint on it, but this packing was under the original coat of light blue. It appeared to be actually under the riveted angle and sheetmetal as well, as if it were put in in ribbon form and sandwiched out when the rivets were set.
The seam around the transom was leaking bad, too bad to even stay afloat for any length of time. I dug out all of that packing, wire wheeled the seam and refilled the gap with a urethane based sealer and then coated the entire inside of the hull with Gluvit and a final coat of light blue paint. The boat came out great, especially for a quick fix, but it's not the first small aluminum boat I've seen with that silver looking sealer or packing in the seams.
Any idea what they were using then?
The stuff is never really attached to the aluminum, it scrapes off fairly easily and doesn't seem to melt with a propane torch, so it's not lead, and lead wouldn't stick to aluminum anyhow. I even considered maybe that is was some sort of gutter seal that someone used, but I've seen too many with this same stuff in it over the years. The way these hulls are built, they rely on the sealer totally to not leak, there are some pretty big gaps on some of these boats. The hulls I refer to are all aluminum with a lapstrake appearance, and the transom panel doesn't really match up to the lapstrake sides, the gaps are what are filled with this stuff.
On one boat, the transom was shot completely from saltwater rot, so I removed it, made up a new rear panel out of heavier guage aluminum, bend the edges to fit inside the hull and just welded the entire transom on sealing it up as I went. It made for a much stronger boat but I am sure I ruined any classic value it may have had down the road. Most of the boats are 10 or 12' V hull boats.
I saw an article a while ago in a boating magazine where a guy fixed a leaky aluminum boat by first cleaning the inside thoroughly,then fitting a plywood floor,under which he poured in foam-effectively sealing the leaks and the bonus was a wooden floor, which he painted with porch enamel and tossed in sand to make it non skid.
Something else that I've seen used to seal up aluminum boats is Rhino Liner spray in bed liner. It seals the leaks, quiets the boat and isn't slippery to walk on.
I'd have done my own with it if there was a place nearby. I like the way it looks and the way it deadens the sound. It don't add much weight and it give the boat a very finished look inside. Not to mention the fact that it most likely protects against corrosion and damage.
My main question is what was the stuff that they used on these older boats? It reminds me of the rope packing that you use in plumbing valves.