My tank was very rusty and varnished bad also. The car had been sitting for years was gas in it of course. I was unable to get a local radiator shop to do mine because of EPA inspections. Both local shops admitted to doing it in the past but had to stop. I bought rust inhibitor and used a piece of chain, sloshing snd rolling with it sealed off. I did this several times and it came out pretty clean. I used red coat to seal the inside and so far all is good. My sending unit looked like a rust ball, I bead blasted, cleaned the resistor, replaced the corks and it works great also.
