I was having some similar trouble. I drained and pulled the whole tank and painted it with POR-15 just in case I has some pinhole leaks. I also knew that I had a leak of some sort near the filler neck that only dripped when the tank was full.

Sealing the tank with POR-15 didn't help, but it turned out that the rubber filler neck was not properly seated on the tank inlet. It appears to have been a factory or later service error. When I cleaned off all the old undercoating and removed the clamp, I was able to fix the problem by re-seating the rubber neck and putting on a newer volvo clamp. No fuel smell or leakage since.