I've cut out the bad drivers side floor pan and welded down an aftermarket replacement. My problem is underneath. There is still a small gap between the old metal and the new metal that would need some sort of "caulking" to seal the seam. Otherwise water will splash up and work its way in between the two layers of sheet metal.

What's the best product to reliably seal up the gap?

thanks
Keni

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isaiah6:8
www.kenithomas.com

I welded entire new floor plans in my 65 valiant convertible.
and encountered the same gap.

I mig welded the bottom as well as the top to fill in the gaps.

I then used seam sealer (available at any body and paint supply house) to cover the welds both on the top and bottom.

I then sprayed the entire bottom with rubberized undercoating.(I used about 8 cans total) I primered the top and then painted it to match as close as possible.

Remember to change your wiper pivots seals.Water getting past your wiper pivots is usually the reason the pans rust out.

If you can't go the "bottom welding" route there is a caulk that you can depend on. It isn't an automotive product. Some hardware stores or building supplies still carry butyl rubber caulking. It is some of the stickiest, stringiest stuff on earth and I've seen it last for decades without hardening.

The name "butyl" means butter, and this is actual rubber butter. But don't confuse the word with greasiness. It is the absolute opposite of greasy. This stuff will stick to dirt!

Because of its stickiness it can be a royal pain to work with. You can think you've never touched it, then you start seeing strings of it, web like, stretching here and there. But there's a simple cure for that. Keep your fingers wet with kerosene and it won't stick to them. I used to carry a bottle of kerosene when I did mobile home repairs.

My floor pans were relatively solid to start, (found a few pepper-holed spots and 3 holes that you could fit a finger thru, after sandblasting top/bottom) Biggest holes were the ones I cut to make room for the subframe connectors that protrude thru the back like 1/4 in. Went to the local scrapyard and bought 2 sheets about 24X42 of sheetmetal, and patched in as needed, welded in solid with a MIG. A couple spots at the seam between front and back floor pans I welded patches top and bottom, think patch sandwich. This is gonna be far from numbers matching, and once the padding and carpet are in, itll never be seen anyway. The local scrapyard charges $5 a sheet. Where you need a little massaging to conform to the existing pan, sometimes you have enuf heat from the edges that were welded leading up to the uneven spot, a couple places I had to use a torch and heat up the patch, then hammer it down to match with the orig floorboard. Make sure you use some spray weld thru coating to prevent rust in places that you can't get to with a spray gun or undercoating. On the gaps you mention, I scraped all the factory seam sealer out of my car, you wouldn't believe all the gaps that the FACTORY left in your car!!!!!! Its a wonder they stayed together as well as they did!