Hi Mate,
Yes this is very common. When you see rust you only see a small part of it. The product you are asking about has a few names and here in Australia I have used a product called KBS but I think that maybe POR15 or similar make a product to seal all fuel tanks. It is not cheap and once the can is opened it will continue to cure so you cannot seal it off to keep for another day. Great stuff when I used it to fix the tractor fuel tank. There maybe a small can available in other products just to suit a small fuel tank. Otherwise get some copper and clean up the surface and soft solder the patch over a large area. The trick to this is to use a sanding disc and clean the area right up. Yes it is tricky but if there are other holes there then you will find them at that stage instead of a week down the track. It is possible to soft solder a copper patch to a steel tank, it is all about the type of acid you use. Research YouTube for that process as it will tell you which acid is best but from memory it is killed spirits.
