Dutch 123,
We had the same problem on our 95 Cruiser. I think the only way to take care of the problem for the long term is to remove the rivets and rub rail altogether and then re-install the rail with through hull bolts, washers and nyloc nuts. I don't care how well you try and seal the top and bottom of the rail with caulking, water is still going to get behind the aluminum rail, move to the rear and settle at the lowest point of the rail as the boat sits on the water and then....corrosion WILL occur to the rivets. Replacing the rivets with ss bolts allows you to seal that hole through the hull with 4200, so any water behind the rail will not penetrate the hull. I tried to seal the top and bottom of the rub rail with silicone caulking at the joint between the aluminum rail and the hull and even though I had cleaned that area well, the silicone soon began to break free and allow water behind the rail. This water would continually leak out of the bottom of the rail through breaks in the silicone, but slowly...I would see water marks for days after the boat was out of the water. Eventually, I removed the silicone from top and bottom which allows any water behind the rail to drain from the bottom of the rail. Pictures of the project are in our album.
Tad