I was installing a bracket for my car stereo subwoofer in my Audi A4.
I was drilling a 1/8" hole in the metal floor of my trunk. I
miscalculated and drilled a hole in the plastic fuel tank below the
trunk floor. Cost to replace the tank was about $1,400, so I started
searching for repair possibilities. Plastic welding was really out of
the question, because I would have paid $800 labor to remove the tank.
I used a hole saw and cut a 3-1/2" hole in my trunk floor. To do
this I cut a board to fit in the trunk over the hole. I bolted it
down with some existing nuts in the trunk floor. I used some rubber
self-adhesives to mark the hole after I bolted the board down. I
removed the board and hole-sawed the 3-1/2" hole. I needed to do all
this because I didn't want the pilot hole drill bit to enlarge the
hole in the gas tank, and possibly contaminate it worse. I removed
the pilot drill, and used the 3-1/2" hole in the board to position and
stabilize the hole saw. The gas tank was so close to the metal trunk
floor that I could only safely partially cut the metal floor. I
carefully cut the remaining metal with a carbide cutting bit and a
Dremel tool. I peeled back the metal well enough to get at the
plastic tank.

I used NAPA 765-1544. Be careful when you go to NAPA. The plastic
tank repair kit no longer says anything about fuel tanks, but the web
site does (see or paste the link below). To search the part in NAPA
online (www.napaonline.com) use the part number without the hyphen
7651544. The description on the web site says "Balkamp Fuel Tank
Repair Kit -- Repairs ABS, Polyethylene, Plastic, Fiberglass & Metal
Tanks". There is also a gas tank repair kit that is for metal only,
but the notes on the packaging are also vague.

http://www.napaonline.com/cgi-bin/nc...2&prmenbr=5806

If you use this make sure you have everything ready before you mix the
epoxy. The work life is very short. Make sure that your fiberglass
(comes in the kit) is cut to size, and the plastic tank is sanded.
The sanding is very important to create a surface for best adhesion.
The epoxy goes from workable and warm to too-hot-to-touch and
to-firm-to-work in what seemed like a few seconds.

The repair worked great for me, and saved me $1,400.

I also inquired into 3M epoxy. The local 3M dealer called 3M, and
they said DP-8005 and DP-8010 are the best for gasoline resistance.
They weren't sure about using it for fuel tank repair, so I never
asked about adhesion to Polyethylene. I may have researched it
further, but then I found the NAPA kit.