Many years ago I got told by an old engineer that I worked with, cattaloy, filler is best, I ran over something with our then mg metro and it put a perfect 5 pence hole in bottom of tank and used filler and didn't even cover hole just filled it as a temp measure to get to work on the Monday and 2 years later was still holding
Did the seam on a mates rover 214 and that was perfect for years, another mates seat Marbella (fiat panda) side & seam lasted years, the trick is it must be fully dry and I cleaned it back to bare metal to see the holes and show others that will appear, Done a few motor bike tanks same way, hope this helps