Does alum boat seam filler shrink and swell in old boats?

There is that (usually black) material along major rivetted seams in most aluminum boats. In older boats, to what degree does that shrink when dry for a long time and swell shut when placed in the water. It seems I've noticed a bit of that temporary leaking in one of my old boats, all of which are leak free once left in the water for the season.

I just bought a 1963 mirrocraft that appears to be in great shape and which was professed to be leak free, but leaks a bit at a seam near the bottom of the bow/stem plate and a bit at a side seam near the waterline (below waterline when I'm in the boat). I'm wondering whether to wait and see, goop it with 5200, or return the boat. When I asked a local dealer to confirm my impression that some leaks may to be due to this dried out seam material, I was told "no, that doesn't happen as far as I've ever seen."

Do any of you know more about this topic?

Thanks!

I would think that is depends on what material was used. My 1973 MirroCraft has nothing like that - just metal to metal as far as I can tell 9and never leaks a drop)

Is this something someone added over the years?

Well, I don't know what it is or if it's on every seam on every boat. There certainly is on this mirrocraft where the two sides of the bottom sheet form the bow and are jointed by a very heavy "stem" piece of aluminum. At least where the laps trakes are, it would seem essential.

I don't have this mirrocraft in the yard right now, so I can't double check the other seam. But I've seen this stuff on a number of boats.

I checked the boat again today. There is an obvious material or "welt" as some say, between the layers of aluminum at every seam. What it is made of, I do not know (thus I don't know if it shrinks and swells at all.

Anyone?

Any pics?

Sorry, no pics. Boat is not at the house right now.

As I investigate this question, I have learned that many boats have this filler material in seams, some call it a "welt" or "welting", and that it was, in at least some instances, made by Goodyear. Those familiar with boot-making may smile at this, remembering that one method of affixing uppers to soles is with a "Goodyear welt" (though it involves no rubber). Other methods are Norwegian welts and Littleway welts. (If only I knew as much about alum boat fabrication-- )

Any additional information or thoughts will be appreciated.

I own a '57 Alumacraft riveted boat, which has that welt material along the bottom. It leaks somewhere, think around the holes for the transducer. However at teh end of a long day of fishing, there isn't enough water to fill an 8 oz. glass. I bought it in January, it was full of ice. Seriously, had about a foot thick layer of ice in it. I put it in the garage to thaw and drain, had to pull the leaves out of the drian hole about once per hour. Took a couple of days, but she finally dryed out. Then I gutted it and remodeled it. Link to teh build in my sig.

My '55 Aerocraft has that black seam sealant between the panels as well as the split fold heading up to the bow.

So far I've never had a leak one from any of it, it's been dry since I've owned it and the previous owner who had it for over 20years said the same thing. He kept it indoors when not using it, and I hate to say I'm spoiling it - but I do as well. Especially over the winter.