My split petrol oil tank also had a slight leak from the petrol to oil side, I used Fedox rust removal treatment followed by Tapox tank sealant, the process was very straight forward, also remembering to use the correct safety gear, gloves goggles apron etc etc. The Fedox left the tank internals looking like new. The Tapox I thought was going to struggle to coat all the tank inside but I removed a good amount after coating, A small air supply lets the sealant go off quicker but you need to be in a well ventilated space. I have had petrol in this now for a few months without any problems.
I would use these again. Easily available via e-bay or other outlets using a web search.
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