One of the biggest mistakes I made when restoring my truck was to try and salvage the old fuel tank. It had a lot of gas sitting in it for 23 years before I got it.
When I drained out the worst smelling gas I ever encountered it also had lots of varnish. I had read somewhere this could be removed by tumbling the tank with some gravel in it.
I strapped the tank to the jacked up wheel on my tractor and let it spin for about 20 minutes with pea gravel in it.
Not only did it not remove the varnish but much of the gravel was firmly adhered to the sticky varnish with no way to remove it. Sure glad new tanks were readily available.
