The factory ran a bead of what looks like expanding foam frot to rear where the two panels come together.
I used some sealer on the same area. I think it is a good idea to keep raod splash from getting up in-between the seams and then into the engine compartment.
Go spairingly as it might be a bugger to remove later.
If your car is an undercoat car that by itself might be enough.
I used tape mastic. It is used on RV's and on metal roofing. It won't harden and is a good seal. 3M seam sealer also works well and is probably easier to find and is what is needed to reseal the interior floors.
I did not use any. I do not plan on driving my car in bad weather. It is a fair weather car. If you do use something, follow prior advice and use something that does not harden.
Closed-cell foam tape/strips, 1/8 x 1. Comes in 10' rolls, I get mine at Grainger, but is available many other places. Less messy than sealer, comes back apart easily, gives a good seal, doesn't absorb moisture.
mine will be a fair weather cruiser like most but i was wondering about squeaks as well as a moisture barrier? Seam sealer seems like what was in mine when dis assembled
it was like curable seam sealer keeps the car stiff and anti squeak. The stuff they used was like glue. It was even on the bottoms of the fenders. I use brushable, paintable seam sealer in a caulk gun