Gravel. Not sand but small sharp rocks. A little water. Sluice it around for a good long time. Then use a good tank coating. Make sure it is fully compatible with The nastiest of the new fuels coming out. I think the POR 15 product should be up to the task, but with E-85 on the horizon, you need to do your homework and give a couple places a phonecall.
It's gonna take more than a week to clean and reline with all the rinsing and drying and application time.
Oh yea, replace the entire fuel line while you're at it. THEN you'll be in business.
