There was some heavy rust and scaling where the rear of the floor pans meet the fiberglass cargo deck. This rust caused the adhesive sealant in this seam separate from the floor pan. This loss of adhesive support created additional stress on the rivets holding this seam together, and a few of them deformed or pulled through. The sealant used is a dense white stuff that clings tenaciously to the fiberglass. The main difficulty is removing this white sealant. I think the only way to completely remove the sealant is to remove either the entire cargo deck or the part with the sealant still stuck to it. Even after removing the rivets, I couldn't get a knife in there at a good angle. A linoleum knife worked best and I was able to remove probably 80% of the sealant. Because I can't get all the sealant out, I'll be using clear silicone instead of seam sealer. Silicone sticks to anything and lasts forever, but it doesn't take paint. It also is not as very strong adhesive, so I'm upgrading the rivets from aluminum to stainless steel.
Fiberglass to Floor Pan Seam
First remove the 14 aluminum pop rivets. In some cases I had to dig out pieces from the seam. Next I used a heat gun and the linoleum knife to remove as much sealant as possible. It helped a lot to sharpen the very end of the knife with a grinding stone. Though you cannot see it in the picture, there is still sealant stuck to the fiberglass. I was unable to get to this without resorting to drastic measures.
Before this area can be resealed, the floor pans have to be rust treated and painted. You could just do the area immediately around the seam, but I did the whole underneath ( see Center Underneath), this includes removing all the old seam sealant. While painting the floor pan, I took a brush and forced it into the seam to get as much rust treatment paint in there as I could. There is a spot weld seam below the rivet seam that should be cleaned out.
I used gray seam sealer on the underneath of the car, to include the spot weld rivet by this seam. After applying the seam sealer, I ran a bead of silicone into the gap between the fiberglass and floor pan.
For the final step, I installed stainless steel rivets while the silicone was still soft. I went back after the seam sealer had dried and painted it black.
SUMMARY: This was very straight forward. I would give it a 4 out of 10 due to the difficulty of getting the old sealant out of the seam. Also, the seam has several turns and angles, making it difficult to move the tube gun while dispensing the silicone.