The car I'm building now sat in a field for 15 years with no gas cap. What what was once fuel in the tank came out of the tank like tar. The tank was trash and I couldn't blow air through the fuel line. I capped the fuel tank side of the fuel line and used a small funnel to fill the fuel line up with sea foam at the engine compartment end. after about a week I took the cap off and all of the crap came out. I flushed it with Acetone until it ran clear.