Epoxy putty will work for a while, fibreglass the same, but if you want a permanent fix, best is to seal it from the inside with Red-Kote fuel tank liner, no more rust, no more worries ....
1983 Honda ATC250R
1983 Honda ATC200E BIG RED (sold)
1984 Honda ATC200E BIG RED
1984 Honda ATC200X (sold)
1985 Honda ATC70 x 2
1985 Honda ATC250ES BIG RED
1988 Honda ATC250ES BIG RED
1995 Honda Frankentrike 50cc 2-stroke
2001 Honda TRX90
2004 Honda TRX300EX (sold)
2008 Honda TRX250TM
1976 Suzuki RV90M (sold)
1967 Vespa Primavera 125
1958 Massey Ferguson 35