Perhaps the previous "Mr fixit" drilled out all the old rivets before replacing any, the entire backing strip may have fallen down into hull.

Or, the backing strip may just be so corroded around the rivet holes you can't see the aluminum, and can't grab it with a rivet.

In that case, you need to drill a "fresh" hole and look for aluminum backing strip. Before taking all rivets out, and taking the strip off the hull, you could put replacement rivets in between each of the old locations. Make sure they're long enough to go through tramp track, hull, and internal aluminum strip. This would then grab the aluminum strip inside, if it's still there.

I would recommend against any repair that does not result in a guarantee of a backing plate, strip or washers of some kind.

Sorry I can't give definitive answer about the 18-2.