If you don't want pretty then do like me. I buy the fiberglass mat with resin. Grind down to metal and lay a coat of fiberglass on both sides or just one side (bottom or outside). I would also take a hammer and see if you can smash the gap together.
Sand and paint. Last time I did that, I bought a can of underneath coating and sprayed my handy work on the bottom to cover fiberglass lol.
I can't tell if it's part of anything structural so I can't tell if you really do need to weld a plat on somewhere to provide some support.

 

The best repair would be to weld it up. Then use a sealant like silicone that is waterproof.

The Gap is too wide IMHO to just fill it.

 

Thanks for the replies. I went ahead and used the deadblow hammer to flatten the bulge from inside the cabin. Now, very little light shines through. I put yellow arrows where I do see a little light shining through in the photo below. Also, the gap from underneath the car has been mostly closed. I don't think welding is necessary at this point. Probably still a little air will come through and this is my main concern It's not a structural issue. Is there a particular sealant I can use?

 

I used an abrasive disc attached to a die grinder for my air compressor to clean up the area. Toyota definitely uses some kind of rubbery sealant in that area. Just don't know what kind. I guess I'll go to the hardware store and try to find something.

Maybe this stuff in the link below?

Dynatron Gray Auto Seam Sealer Caulk 12oz

Shop for Dynatron Gray Auto Seam Sealer Caulk 12oz with confidence at AutoZone.com. Parts are just part of what we do. Get yours online today and pick up in store.

www.autozone.com

 

Welding will not hold on rusted metal.
What green said is your best bet. Stacked layers of fiberglass. I'd say, as prosthetist, who works with laminations, 5-6 layups per side. Would have been nice to introduce vacuum, so that it sandwiches everything real tight together, but that's rather hard to do, though can be done. Shop vac can provide suction.

 

I wound up using that body Seam Sealer caulk. It seems to be simillar to what Toyota used. I'm not very artistic applying the stuff on, so it looks messy, but it's underneath the car and also underneath the carpet, so I don't care. I just wanted this thing sealed up:

I put a light underneath and at night time I could not see any light coming through the floorboard, so I'm gonna call this one "fixed". This is a beater car and I'm just trying to keep it rolling. Thank you, guys, for your suggestions.

 

I think you did a great job at repairing that mishap. Stopping any exhaust fumes from entering the cabin is the biggest concern. Good job!

 

I think you did a great job at repairing that mishap.

Thanks for the positivity, but man, it looks pretty ugly, ha ha. I'm gonna need practice on the aesthetics. At least it's not in places that show.

Stopping any exhaust fumes from entering the cabin is the biggest concern. Good job!

Absoultely, especially since this car originally came from Wisconsin and it is rusty, so the exhaust is a concern. I'm gonna check for leaks today if I have time after putting in the carpet and console.

What I realize now is that I should have applied some of the seam sealer before I pounded down the floorboard. I did think about it briefly, but didn't do it. Now I wish I had, but I am confident it is sealed up. And for any beginner DIY people here, make sure you are jacking up the car from a good solid spot that can support that. This is what happens when one doesn't know that.

 

If you find any gaps or small holes, I would highly recommend using POR-15 with some fiberglass cloth. Has worked very well for me on different vehicles

 

POR-15 is a rust converter. Seam sealer is a good choice, A urethane exterior caulking would have worked real well, sticks better than silicone and is a bit harder too. Some are self leveling depending on temperature. You don't want anything that dries hard.

 

The seam sealer you used would be my choice. You should be fine.

Mike

 

As much as I like JA, we never have time to do things right but always have time to do over.
That caulk will let go the very moment rusted metal flakes off. With it. When you make fiberglass patch, you grind metal down to rough, and overlap about several inches along perimeter. On both sides. THAT will hold for years to come.
I'm sorry, man. Just cuz I like you.

 

Thanks, will check it out again when he comes back to visit in a couple of months.

 

Yeah.. it's floor board, man. You don't really want it to let his foot out... when you go layered fiberglass on both sides, it's like kevlar. PITA to do, as fibers stick to wet gloves, but very rewarding results.
It scares me, every time I see "floorboard" and "gap" together.

 

back when I was growing up, in the late '60s and '70s, every old car had a rust through hole in the floorboard, either front seat or rear seat, with only the carpet keeping you "in"

Most people shrugged and lived with it until they needed to replace the car for mechanical reasons. Luckily, metallurgy and rust protection have come lightyears since then. A little crack between the panels like shown in the photos is of no concern, and sealing it up like shown looks like it will keep any water out. If strength is a concern, apply some rivets by drilling through the two flanges where the spot welds got separated (and seal them up with more of the goop already used).

While there'd be no real downside to trying to go over it with fiberglass I don't see much call for it, and doubt that it would get any useful structural grip to the surrounding metal without a lot of effort made to get them successfully bonded somehow.

Norm