When building my Viper, I air pressure tested the fuel tanks in water & repaired a small leak. When the build was finished, I filled the tanks with fuel. After a couple of hours the bottom of the fuse was wet with fuel. I stripped the tanks back out (a real pain in the *****) & air tested them again to find more leaks at the seams. I have sanded the seams & sealed them with cyano & then thin fibreglass cloth & epoxy.