Strip and respray, starting with several layers of grey primer and wet-sanding with 600 grit. The idiot former owner used the wrong paint, probably rattle-can stuff. Good paint is not affected by petrol, modern or not. My favourite tank paint is brush-on enamel, but it takes some technique to get it on smoothly. I mount the tank on a long piece of timber so it can be rotated, then flow the paint on with a very good brush while rotating it. When it's covered, I keep turning it and start a heater to set the paint. Rotating makes it impossible for runs to get going. As soon as the paint is stiff enough not to run, turn the heat off, turn it for a few more minutes (to make sure) and go away. Needless to add, there should be no flying insects in the room. Next day, put it in the sun if you've got any. The paint will be soft for a week or two, so avoid scratching or knocking it, and no petrol or other solvents.
