Can anyone recommend a product that would be a reliable, long-term solution for sealing a cracked gas filler neck?  The vehicle in question is my friend's 10-year-old Hyundai Elantra.

It would depend on the demands of the application, flexible or rigid, but I would check the specs for fuel resistance specificity.  Looks like it's a hot topic, several options popped up with a search.

My only real-world experience is sealing a pinhole leak on the bottom of a Pathfinder gas tank with JB Weld's Steel Stick product.  It was a defective spot weld.

https://siliconesolutions.com/ss-300.html

https://www.permatex.com/products/gasketing/gasket-sealants/permatex-permashield-fuel-resistant-gasket-dressing-flange-sealant-2-oz/

https://www.jbweld.com/product/steelstik-epoxy-putty-stick

https://www.jbweld.com/projects/gas-tank-repair

I have used this before on an early 80s Ford fuel tank that had pin holes near the mid line. It worked great and lasted as long as I had the truck

https://www.amazon.com/Seal-All-380112-Contact-Adhesive-Sealant/dp/B008VK0JS4

2 Oz Eclectic Products 380100 Seal-All All Purpose Contact Adhesive | Adhesives, Hobby & Repair, Specialty | Paint Supply

(I can't see the price at amazon as it can't get shipped to my location, is the mentioned price in my link sheaper?)

9 hours ago, Patcon said:

. It worked great and lasted as long as I had the truck

And that was 2 months?  😉 

Edited July 16, 2024Jul 16 by dutchzcarguy

I really hate firms like Amazon! They abuse workers and kill the market for small firms.. so don't buy it there if you can..

Amazon= abuse!

Edited July 16, 2024Jul 16 by dutchzcarguy

Im not a huge fan of Amazon but they are becoming the distribution network for all businesses. That ship has sailed I think.

I had that truck for another 4 or 5 years probably. The only thing  i don't know is how flexible it is after it's cured

Red-Kote seems like a good choice too.  Made for sealing gasoline.

http://damonq.com/red-kote.html

RED-KOTE®

Red-Kote is an internal fuel tank liner designed to seal leaks and prevent further rusting. As a sealant, Red-Kote excels at sealing the often hundreds of pin-hole leaks that occur along seams or where straps wrap around the tank. Once in place, Red-Kote forms a very tough, flexible membrane that will never crack or flake as many competitive products do. Red-Kote will not plug lines or cause engine damage when used properly. Future rust will be prevented because condensation will not contact the metal. A partial list of additives that Red-Kote is resistant to includes ethyl alcohol, methyl alcohol, toluene, methyl tertiary butyl ether, isopropyl alcohol and tetraethyl lead. Red-Kote takes approximately eight hours to dry