I just started working on an 85 cabriolet it had been sitting for many years the ploblem i ran into was when i took out the fuel sending unit, besides being covered in brown sticky varnish and not working, I drained out as much fuel as i could i found the bottom of the tank coverd with a black sticky sludge can this be cleaned with fuel additives like sea foam or do i need to pull the tank. thanks i advance for your help