What seam sealer?

I'm finally getting to repairing the rusted fenders on the Infiniti, and was going to get the passenger side one done today- until I found out the seam sealer I already had was dried out. I've been trying to order more- but all of them say they are non-hardening. Is using a permanently elastic one going to work? The plan was to use the seam sealer to bond the parts together and reinforce it with a few welds. 

I'm planning to use a 2K seam sealer on my '61 Apache.

3M panel bond, surprisingly therapeutic on your hootus. 

*not actually therapeutic for your hootus, 0/10 do not apply. Results will not vary. Seek medical attention if applied. 

I have succesfully bonded patch panels using POR-15 patch. It's like POR-15 but thicker. I just prepared the panels like I was welding them and glued them. After that I painted with rubberized undercoating. Five years later it's still holding and there is no rust.

I also use the black Pro-Form sealer the local NAPA carries. It doesn't totally hardens like the POR-Patch. Last time I did a repair I wanted to use that over the welds and like you, I found out the tube was dry. I had POR-Patch so I used that. We'll see how it does in a few years. It's written on the tube that you can use it as a seam sealer.

If you want to glue panels, use panel bonding adhesive.  Seam sealer us fine, but its designed to stay soft.  

I've used eastwoods panel adhesive.  Worked fine but it seemed a lot like jb weld to me!  

I used Silaprene to glue my miata panels in paired with rivets.  Still seems to be holding well almost two years later:

Here it is after paint:

3m panel bond is my go to. 

Thanks everyone! Sounds like the 3M panel bond is the way to go. It looks like there are several varieties of it- which would you recommend for bonding in a panel in the wheel well/fender? 

Check the spec sheets.  You probably want one that will finish hardening completely.   I have one that is purely anaerobic, so it won't ever harden if its exposed to the air.  Another hardens very nicely in open air.  For a big ol' rustbucket, you might want to use it over some remaining exposed rust.  It works wat better for that than plastic filler.  Not that I've ever done anything like that.

Oh, wait a minute...I've done that a whole lot.

3M panel bond is used heavily in the OE sector to join and reinforce panels. I believe it can also be used to assist in structural rigidity in certain structures. Its expensive though.

I ended up ordering a 200ml container of 3M panel bond, an applicator gun for it, and a tube of POR-15 Patch (because I need to seal off a few places I had to grind out that weren't bad enough to patch in a whole new section as well). Last item may not get here for a week (applicator), so I'm going to get the driver's set prepped so I can weld/bond both in quick succession so I don't waste any of the panel bond. Not cheap- but if it keeps the QX4 on the road for a few more years its worth it. Thanks!

So, a followup question on this- when using the panel bond to attach a section of metal that overlaps, how is the best way to apply it? I would assume to apply it like glue around the edges of the top (or outermost) part and press and clamp it so it both is between the parts and also presses out a bit and over the edge of the outermost sheet of metal?

In reply to Ashyukun (Robert) :

Many times you want to butter both sides of the joint before you stick it together.

Made the progress I wanted to last night, so this afternoon will be doing the actual bonding. Unfortunately the area I have to replace on the driver's side is so extensive there was just no way to get overlap on both the inside and outside of the wheel well I'm repairing, both because of the complexity of the area and because the piece I cut out from the junkyard car has some issues too (but is way better than what mine was).

I'm going to have to fill in some gaps on the inside of the wheel well- anyone have any good, inexpensive filler suggestions? 3M Flexible Foam seems to be pretty much what I'd want, but for as little as I'm likely to need it's pretty expensive- anything more budget-friendly, ideally that I could find locally (actually apparently can get 3M foam from O'Reilly, but at like twice what it would be if I got it from Amazon...).

Since it was rather warm midday today and the panel bond cures faster in higher temps, I ducked out to take care of the work over lunch instead of in the evening. Things went well enough- not perfect, but this was never going to be anywhere close (nor did it need to be).

Passenger side was first- it was small enough that I could bond both the inside and outside, and tried tack welding the corners of the wheel well angle. Seems to have worked well enough.

Drivers side was much larger and more complex. I had actually intended to make a few tack welds to it as well to hold it tight, but the advantage of it being so large of a section was it was a lot easier to clamp it in place so they didn't end up being necessary.

If I have the time tonight (unlikely since I need to make up the time spend in the garage midday at work) I'll use the POR-15 Patch to seal up around the edges and the parts I had to cut out from the new parts and where I didn't bother putting a patch on in the first place. I can probably do it tomorrow easily enough, but would love to have it on and curing for a day and a half before likely taking the thing out driving in the rain Friday...