Here's the process I did back in 2012. The rivet still has not leaked since and was on the bottom of the tank.
From 09/2012.
This is an update on my original post.
Seven days into my fix of a weeping rivet, problem solved...
As stated above, I used a a syringe with a 1/2 diameter barrel. I cut & removed the needle mount to expose the 1/2" opening. I made sure the cut was clean & symetrical to allow a perfect seal when placing over the skin & weeping rivet. I used Pliobond as it has a slow cure and is chemical & gas resistant.
To begin the process the fuel level must be below the weeping rivet.
1. Remove the fuel cap.(this will allow a pressure variant), although the vent does too
2. Load a small amount of pliobond into the syringe.
3. Place the opening cut end over the weeping rivet and make sure you have a solid seal against the skin.
4. Depress the syringe. You should feel resistance if you have a solid seal against the skin & providing the push.
5. Repeat step #4 to insure enough pliobond consumes rivet.
6. Let cure for 48 hours to ensure proper seal.
Others have suggested green loctite, which may be a better product. But, I happen to have pliobond that I had previously used & was recommened by Lycoming for crankseal sealing.
Hope this works for your situation. It's a low cost alternative to what others have suggested. Now I just have to come up with a way to remove the blue fuel stain.