My 97 Maxima has developed a gas leak which I traced to a hairline crack in the plastic fuel gauge sending unit, which acts as a cover for the fuel tank. The part seems rather expensive at $130+, so I want to try mending it.
Can you please recommend a glue that would work and hold up to gas and alcohol?

Long time ago I used leftovers from a glass fibre body repair kit, which is a polyester resin. IIRC that wasn't even supposed to work then (though it did) and petrol formulations have allegedly got more aggressive in the interim.

Now I'd probably try epoxy. If you can wait, maybe whatever you try you could test its petrol resistance first to avoid contaminating the surfaces with something ineffective, in case you find something better later on.

PS 870 B½.

It's expensive though...

VersaChem, Permatex, and others make gas tank specific repair kits. Since your looking to fix a plastic part instead of the steel itself, I'd look into a plastic specific bonding epoxy that is solvent(or at least gas) resistant.

I repaired a cracked filler tube neck in a 1998 Ford contour, leak stopped. That was years ago and it never leaked again.
They call it PL S30 nowadays. Gas has changed since then. I would still use it today.
I used Black Loctite PL Polyurethane roof and flashing caulk. You can put on a layer about 1/8 to 1/4 in depth.

JB Weld makes a variety of epoxies that should work.

+1

3M 8802 gas tank sealer

been there done that. works great.

Playing with small engines and chainsaws for a hobby I have often found problems with cracked leaking gas tanks. I tried alot of epoxies and sealers, all of which were short term solutions. Nothing worked for long. What I have found that works forever and quickly is a soldering iron or wood burner. Melt the plastic back together. I just repaired a plastic chainsaw tank with a large crack and hole. Works like a charm. Not dangerous and pretty much permanent.

Thanks for the replies!
I'm favoring J-B Weld at this point. Should I go with the regular or the WaterWeld?

@Panzerman - I assume you just melt a bit on the surface of the plastic, and spread it over the crack?

I'd get this - but likely any JB Weld will work.....

https://www.amazon.com/J-B-Weld-8217-AutoWeld-Repair/dp/B00R2CDUWE

You need to actually dig the tip into the crack, then force the material from the sides to fill and level.
It can work, works good on some plastics. Some it just does not work well, you get a weak result.

You can buy plastic welding rods, but the plastic types must match.

Don't mess with heat and a gas tank.

It all depends on the type of plastic you are dealing with. Some take glues well, some don't.

First choice for me would be a Putty Epoxy ... easy to apply, works on water wet or gasoline wet areas, and impervious to gasoline and alcohols. If you're dealing with certain plastics that won't glue up ... your option at that point is plastic welding (basically a form of heat melted plastic bond). Try Eastwood for the relevant supplies.

That may not work either ... some plastics get brittle if heated. But those are your basic options.