I fixed our tank a few years ago with chemical metal, as I had some pinholes in the top section. It lasted ok, but started to fail when the tank was left full for a period of time.
I have just done it again, and this time used the POR-15 petrol tank repair paint. I could not apply it to the inside - no way was that sender unit coming out - so I cleaned up the outside, painted the outside, then covered that with a cloth, and painted that too. Once it was all dry, I gave it another coat. It has formed a pretty rock hard shell, and I reckon it'll do the job 
