I have had my 70 Roadrunner upside down on a rotisserie doing the torque boxes, frame connectors, etc. The last thing to do is on the rear of the rear inner wheelwells. There is rust through on each side in one area. I'm going to cut that out and put in a patch. These inners were replaced in the past by a shop for the previous owner. They were never welded to the lip but rather sort of poorly crimped in place. Hence the rust through.
Once I put in the patches I need to seal up this void. I'm looking for a good way to do this. Keep in mind that the car was completely repainted and I will not be doing that except for the bottom. So I either need to use some sort of gap sealer or possibly use a fiberglass laminate like we did in high school (40 years!).
Does anyone have a product or procedure in mind that may fill this bill?

Thanks for any info

Poorly crimped in place? What does that mean?If you can prep the area then I would use panel adhesive.

On my E I cleaned out the trunk extensions as best possible. Mechanical means like stiff wire and shop vac. Then high pressure water. Then let it dry thoroughly and applied Eastwood waxy rust proof from the inside until it dripped out the water drains. This stuff will seal small gaps (eg. 1/16"). Bigger gaps will need additional help I guess.

Not sure exactly how to describe it, They were welded at other points, some good, some bad. I can deal with that. But the lip is like it didn't match up. So they used some kind of clamp on it with something to spread the load on the outer fender lip but bending the inner fender lip a little section at a time. There is no seal at all on the outer part all along the lip. Some places a 1/2" gap. The money must have been on the paint job afterward as that's done really well.
It's like this car was made to win the show at Billy Bob's grocery market. Hence why I have it upside down.
Can panel adhesive fill gaps??

Panel adhesive will fill gaps to an extent. Better than welding where you could have a heat-distortion problem.

For a gap that big, I would suggest polyurethane seam sealer... which is pretty much the same thing as high end polyurethane caulk at the home improvement store. A 1/2" gap will need a backer... foam rod also from the home improvement store.

Funny, I was looking at Dap Extreme Stretch Elastomeric sealant at Home Depot today. Didn't think about the foam rod though.

Thanks for the tip!

We buy a cheap hand pump sprayer. Fill it with POR 15. Put the nozzle down in the crevices and pull the trigger. Put paper down as it will drip out. Let it harden up. We've been doing them like this for over 15 years. Any area you really can't get to. Doors, cowls, etc. My tow vehicle is a rust free 1995 D3500. It has the original doors done this way, no rust. Living in Michigan it's a miracle.
Doug

i would highly recommend against using a foam product of any kind ! the foam WILL retain moisture due to condensation, and WILL promote rust issues. a lot of dog leg and rocker issues due to the OEM's use of foam to reduce NVH. just a heads up.

Any thought of putting a foam backer rod in there would be accomplished with it saturated and surrounded by a good elastic sealer.
My hope is that I can manipulate the ends needing to be sealed close enough to not require it, so using the sealer alone would work.
My other thought is to cut off all the bends for better access to clean everything better and use some fiberglass cloth and resin to bond to both surfaces and bridge the gap. In this way it would look cleaner. The thing I worry about is if any vibration or flex between the 2 surfaces would cause cracks and leaks in the fiberglass. So still thinking about this.

Fiberglass does not bond to metal forever.
Automotive structural adhesive does.
Just close the gap - if I understand it, you're talking about the outer wheelwell and the quarter panel, where the flange is - and it will seal as much as 1/4" gap. The 3M 8115 & 8116 are excellent products; same stuff either #, one # is the GM approval.
If the car's never going back to OEM stock/restored, you might want to trim the flange back or roll it upward for tire clearance.

I think I can work with 1/4". And thank you for the actual product recommendation. I'm also thinking if I should roll them back. 295/50 fits good on there before I started the project but now sure where I will end up tire wise in the future.

You can buy a tube & applicator off Ebay. Another thought is the front of the wheelwell, where any tire over 28-29" typically is too close to the opening flange. I've dealt with that by pie-slicing through the quarter & wheelhouse (up to the curve), moving the flange forward, and bonding. Adds another 1.25" on a Duster.

I have done a few of these repairs on various vehicles with the 3M products as mentioned. Clean, clean and did I say clean some more ? Follow their directions, the results are excellent. The usage of an internal cavity wax after the adhesive has been applied and cured is also a good one.